> "no one person is truly responsible for progress or breakthroughs, and everyone's contribution matters", which is ridiculous, and the motte is "no man is an island, and all breakthroughs rely on the actions of others", which is obviously true.

I think you are missing here the real core of the "Myth of the Lone Genius" argument which is not to claim that lone geniuses don't exist (e.g. Ramanujan, far more than some of these other examples) but rather to reject the argument that progress is mostly made by them, or can be best understood by focusing on them.

The argument boils down to: everyone's contributions are clearly not equivalent, but the vast majority of real progress is a collective effort; and that by focusing on individuals we are less efficient at learning what is effective.