If that actually fully blocked everything coming from an instance, I would agree with you. But it only blocks posts in communities on that instance, and that is not the problem here, those communities see no activity anyway.

One person on that instance is flooding New Communities (and elsewhere) with their garbage, at one point there were seven of their posts on the first page alone and yesterday a bunch of obnoxious comments on unrelated posts. And I can’t just block them because they make a new account for every tedious gimmick. Many of them are getting banned now at least, maybe they’ll take the hint.