It's not that I really disagree with this, I just think it's mostly moot. Overall, it's splitting hairs in a somewhat odd way, and especially considering the context of OP, where their friend is pressuring OP to vote Democrat, it just doesn't apply. It's almost obvious that what we are talking about here is not some secret schroedinger's voting booth vote that no one else knows about, but a vote that is announced and therefore is a form of, as you put it, public endorsement. If someone secretly votes Kamala without telling a soul, especially if they're doing real community work that benefits people in material ways, then sure, none are the wiser and the actual work that person is doing supercedes whatever asymptotically minuscule effect a single vote has. But then we're almost getting into "If a tree falls in a forest and no one is around to hear it, does it make a sound?" territory.

Though that's not really what we're talking about here. Going back to my analogy above about a friend supporting the murder of your family (and to be clear, I know you're not the same person I was responding to with that analogy). If you are completely and forever ignorant of your friend's support for that person murdering your family, and their support of that murderer doesn't actually change the ability of that murderer to do any more harm, then of course, you would have zero reason to think or feel negatively about your friend and no material difference is made. I would still contend that your friend is a shitty friend and a shitty person, but again, "if a tree falls in the forest..."

But that also does bring up another aspect of this. The person who voted blue (or in the analogy the friend who secretly gave their insignificant support to your family's murderer) still themselves know what they did. So it matters to them. And while it may make zero difference to the outside world in that very moment or by that specific act, it still "matters" in the broad sense depending on their reasoning. If a person votes blue, or red for that matter, so long as they're voting for fascists, is it because they do feel that the benefit to themselves outweighs the genocidal harm that candidate has done and will do? Is it because they laughingly did it in a cynical fit knowing that their vote doesn't mean shit, though they actually despise the person they voted for and recognize they won't benefit from that vote? If it's the former, then that person is still someone who can't be trusted to do the right thing, even if no one is aware of it. If it's the latter, then sure, but I think their jokerification is approaching worrisome levels even if no one else knows. To put it one last way, if someone is secretly a racist, but never expresses it and only does things that positively effect the people they hate, then "no harm, no foul" but I still think that in the real world, a person who is racist will always tend to behave in ways that have negative effects in the world. And so too with secret Harris voter. No one might hear that one tree, but there is a near certainty other trees are going to fall when people are around to hear it.

Edit to add:

In short, I'm with @frauddogg@hexbear.net on this one.

I'm not going to pick arguments over votes in the presidential election

I am. I want to know exactly who decided that in their personal calculus, that they could accept the genocide of another sovereign group of people if it meant the security of their own rights-- because when it's my turn on the sacrificial altar, it'll be those same coons, crackers, and assorted miscellaneous misleaders holding the knives.

I want to know who I can't trust turning my back to.