It is gatekeeping, but gatekeeping in the way of "Stop corporate offices in this town" and not "Stop people who we don't think worthy from getting in".

As for why blocking threads is good, please google "Facebook Cambridge Analytica", "Facebook russian accounts" and "Facebook fake republican accounts". Also: https://www.nbcnews.com/tech/tech-news/facebook-knew-radicalized-users-rcna3581

Please don't treat facebook like a "decent corporation which only committed honest mistakes". It sold users' data to corporations, to the Kremlin, allowed users to be specifically targeted by extremist right-wing propaganda and spread disinformation about various international affairs.

Furthermore, there is absolutely zero guarantee that Facebook won't scan OUR posts for training AIs.

It's a known bad actor. Allowing Facebook into the fediverse would be as ludicrous as allowing Russians to live and establish bases in the US during the cold war.