If Marxism is a science, then it must be testable and falsifiable. At least, that's my understanding of the scientific process. It's why Freud for example is considered unscientific. This theories have a lot of wiggle room, where any result can be turned into a proof or explained away.
Isn't this what we do with the various leftist projects? I don't know, I'm just confused why it is called a science and not, like, philosophy.
I think that's the wrong way to think about science. If you do go down the road of looking at Marxism as science, you'll need to go to more modern understandings of science than old Popper falsifiability crap.
Though I think that consituting Marxism as science at all like that is wrongheaded. The point of Marx's scientific socialism is that it was logical and materially based in its attempt to change the world, unlike the utopian socialists it criticized (whether it does this successfully or not is up to debate, I lean to yes with caveats).