they’d only have to make a profile on a different, normal server my “All” feed would still be full of garbage-tier content that people typically expect and post on Meta’s services
If they make a profile on a normal server then your feed will be full of that same content you don’t want. You’re trying to exclude users, not meta itself.
Besides, you keep taking as though they’re federating with Lemmy. They’re not, they’re federating with mastodon. Having mastodon posts show up automatically in your Lemmy feeds is unusual. Kbin, maybe.