While what you said isn’t untrue, .ml does Bill itself as a general purpose instance. Also, not all the replies are from accounts on .ml.
In a more general sense, I always felt that only reading sources that alligned with one’s political alignment narrowed one’s perspective.
I don’t think the average user thinks much about the platform they’re on, and about who controls it. I think they go to wherever most of their family/friends are.
Also, those platforms are firmly in the mainstream, the alternatives aren’t really - you’d have to actively go search for them. People just aren’t likely to do that, I don’t think.