The thing that isn't being mentioned is that Google allow apps that make "reasonable" attempts to block content that violates their anti-hate speech policy. Reddit has shown that they're willing to ban the very worst content. No idea about 4chan.

In the case of the Fediverse apps they can't block anything because firstly there's no resources to police it, and secondly it's kind of the whole point of federation to let the user see what they want without getting in the way.