The point isn't to disprove determinism, and definitely isn't to do so in favor of free will. The point is to achieve a dialectical materialist understanding as opposed to a mechanical one.

In your previous thread you say this about "sentience":

yeah, but what is it? how does it have free will? isn’t it just regular matter subject to conditions, not able to make decisions.

Firstly, I think there's some confusion about free will and will. Free will is an idealist notion that essentially our minds can operate above or outside of the laws of physics. That is clearly false. Just will, on the other hand, doesn't have idealist connotations. I think that's an error your interlocutor made in that thread, or a general error of not defining the terms discussed. An error I think you've made here and in general is opposing the two positions of mechanical determinism and free will in a dichotomy as the only possibilities.

I'm partial to @redtea@lemmygrad.ml's thought about there being a category error. I think your mistake is in thinking that everything is infinitely reducible into smaller parts, and also without loss of context. In more general terms, I don't think you've fully grasped dialectics.

We know from dialectics that relational properties are very important, and abstracting things doesn't let us analyze them properly. I think you're missing a key concept of dialectics when you assume that the parts that make up the whole are ontologically primary and exist separately from the whole while still being the same parts that make it up. I mean this in the sense that different bits of matter make up us, so from their properties you assume it's clear that no will exists because atoms aren't sentient. Your mistake is in not recognizing that our sentience is a property of matter. Not of abstract matter in general, but of the specific organization of matter which results in us. You say "regular matter" as if some other kind of matter would need to exist for sentience to exist.

A simpler example can be made from the properties of water. A single molecule of water doesn't have surface tension. Following your mechanical model, we cannot really explain how water, when organized in a larger body, does. This is in general a fault of the Cartesian reductionist model which predominates in science today instead of dialectics. The concept which is usually used here is that of emergent properties, but it doesn't really explain anything by itself. Dialectics on the other hand doesn't even see a problem here to explain because a water molecule on its own and a water molecule in a larger body of water are two different things. The parts of the whole don't exist separately from that whole as its parts.

The properties of the whole and the individual parts of that whole don't exist separately from their interactions as parts of that whole. These properties only come into existence from the interactions of the parts and the whole. By simply studying individual water molecules, you would never discover surface tension. Parts interact with each other and with the whole, and the whole interacts with all the parts. A common example of this in Marxism are the base-superstructure relations. None of the components of either the base or the superstructure exist on their own, they are parts of the whole that is our society. The economic base tends to have a stronger influence on the superstructure, but the specific relations are constantly changing.

Here's a quote from Sayers' critique of mechanical materialism:

This is the dialectical account of history given by Marx, and it differs entirely from Cohen’s mechanical interpretation. The differences are clearly spelled out by Engels in the well known series of letters that he wrote towards the end of his life. In them he insists that the economic system and the superstructure are not simply the immediate and direct products of the prevailing form of production. Although their character is certainly conditioned predominantly by the development of the productive forces, it cannot be reduced to this factor alone. On the contrary, the economic system, for example, acquires its own distinctive character and its own inner dynamic. Through the division of labour, trade and commerce become areas of activity increasingly independent of production. They acquire, in short, a degree of “relative autonomy”.

Where there is division of labour on a social scale, there the different labour processes become independent of each other. In the last instance production is the decisive factor. But as soon as trade in products becomes independent of production proper, it follows a movement of its own, which, while it is governed as a whole by production, still in particular respects and within this general dependence follows laws of its own: this movement has phases of its own and in turn reacts on the movement of production.

The same is true, even more clearly, of political and legal institutions and of art, religion and philosophy. None is purely “functional” to the development of production. Each of these spheres, while in general being determined by the development of production and by economic forces, has its own relatively autonomous process of development, its own relative independence. Each affects the others and the material base.

Political, juridical, philosophical, religious, literary, artistic, etc., development is based on economic development. But all of these react upon one another and also upon the economic basis. It is not that the economic condition is the cause and alone active, while everything else is only a passive effect. There is, rather, interaction on the basis of economic necessity, which ultimately always asserts itself.

Another way to put this is through the constancy of change in dialectics and the build up of quantitative change into qualitative leaps. You cannot simply "go down a level" of quality and look at the quantitative aspects of the lower level to understand everything in the higher. The surface tension example can again be used here.

Taking from all the points above, we are active parts of the whole, our societies, our history, and we have constant and mutual interactions with each other, with the other parts, and with the whole. Our wills and choices (still far from free) do matter here very much and we do make the choices. Our consciousness is a key part of the process of our history, as is also seen in the notion that freedom is the recognition of necessity. Therefore, to deny our conscious will (not free will, which is idealist) and its effects is a mistake, and akin to saying that water doesn't have the property of surface tension because an individual water molecule doesn't, or arguing that social constructs aren’t real.

This doesn't "disprove determinism" in general, and it doesn't seek to. It's just a proper contextualization of phenomena and processes. It does highlight the limitations and mistakes of mechanical determinism. Out of the specific interactions of the organizations of matter that make up us, come the properties of consciousness, thought, will, etc. Our will is simply a property of matter organized in a specific manner. There is no need to assume any metaphysics or idealism to describe our wills.

Another quote form Sayers to hopefully round this out:

Even in the realm of purely inorganic, physical phenomena, the mechanical view is an abstract and metaphysical one. It portrays physical objects in an idealised fashion, as unaffected by their relations.

[...]

Of course, the mechanical outlook has played an extremely important role in the development of the scientific understanding of nature, and it is not my intention to reject such methods and assumptions altogether. The error comes when such methods and assumptions are made into a universal philosophy and emphasised in an exclusive and one-sided fashion. Their abstract character is forgotten and they are employed as though they alone formed an adequate basis for understanding reality. The result is an abstract and metaphysical view of the world.