Do you have a source!

Blaze had calculated that at some point. It was quite awhile ago so that could have changed radically since then but... based on all of my personal experience things seem to mostly be moving in the opposite direction. e.g. as mander.xyz and slrpnk.net are both down (hopefully just temporarily), and smaller communities struggling mightily hence people remain in the biggest ones.

One example is !tenforward@lemmy.world retaining 5.77K MAUs compared to its offshoot !tenforward@piefed.world with just 0.08K MAUs. And I mentioned others like !nonpolitical_comics@piefed.social previously. You likely see this happening all across the entire Threadiverse. We cannot support much niche content so it generalizes instead. The trends towards centralization are not being counter-balanced as much as previously by trends away from that.

It does seem believable to me though, that separately from all MAUs, the number of existing communities that people post into are concentrated on LW. Again, there was a huge shift to try to spread them around, but that was before the Flotilla began threatening to split the Threadiverse in two in consolidation with anarchist.nexus, and I for one tend not to post into communities located on dbzero anymore (I am not like morally opposed to such, but I just don't try to prioritize it as I once used to).

doesn't appear to be actively antagonistic.

That is probably due to your having blocked a lot of it? But if you narrowly mean between AN & DB0 vs. LW then yeah to that much. Although I recall reading in the DB0 meta community that the call to defederate from LW was postponed: (1) for something like half to one year, for preemptively defederating then - i.e. this calming down period was intentional yet a vote as to whether to continue was still on the schedule even in the heat of it all, and also (2) separately from that, virtually a guarantee to happen if any single member of the Flotilla ever gets defederated from, I think this one not merely by LW but by any instance whatsoever, anywhere. That is no mere threat about some future vote that may or may not happen - that is outright policy, a settled matter, iirc.

Again, not that any of this stuff is easy to keep up with, even by us here, to say nothing of newcomers.

All the "non-aligned" instances (not neo-liberal, not ml, not anarchist) is like 50% of the threadiverse and they would still see everything.

That assumption might not be true. If the Flotilla defederates not merely from LW but from any instance that remains federated with it, then the schism would be wide indeed. But we'd have to read more the Flotilla policy to see (again) their stated aims. I forget the likelihood of this being true, but it's definitely nonzero and more than just a vague conspiracy theory - whether it actually happens or not, people were most definitely clamoring for exactly that to occur, and it was not a fringe opinion iirc. Then again, after the cooling period, things probably will look different, so most likely the most extremist opinions will be drowned out by those wanting the greater convenience of remaining connected.

I will now go read your post:-).