Weak moderation on many instances -- including large ones like mastodon.social -- is a big problem, but I wouldn't say it's the biggest. Black people even on well-moderated instances get plenty of racist abuse -- the moderation tools are horrible, and basic tools that peopl on Twitter have to protect themselves don't even exist. Agreed though that many fedi instances do have stricter rules and make a real effort to enforce them ... that's a good thing!