Anecdotally, I disagree with that characterization. Though, when you have the most people period, you do, somewhat intrinsically, tend to have the most of all "types" of people.
That said, I need convincing that lemmy.ml, who they are still federated to, has a meaningfully "nicer" community.
It was less about intangible "concerns" and more that most of their time spent moderating was from users of this instance and since they're strained on time, it made sense to simply defederate until the necessary mod tools exist to make things easier.