But in your scenario, all the communities have different niches: GP, dirtbike, Harley

In the article, the scenario is taken about identical communities, like !movies@lemmy.world and !movies@lemm.ee

Of course if at some point nazis invade people should migrate, but that doesn't mean you should keep both active at the same time (also, nazis could invade both in parallel, not sure how that solves the issue here)