I think we're talking about two different things. I'm saying that servers ultimately choose what they receive. People worry that Meta will flood Mastodon with unwanted content but content has to be invited in. Although it's more accurate to say that users have to be invited in, like vampires, to serve content. People seem worried that federating means inviting in all the vampires.

When users on server A follow a single user on server B, it doesn't matter if server B has one user or ten billion, server A receives content from one user. The only way to receive all content from a server is to have at least one person following every user on the remote server.

So Meta can't flood Mastodon with unwanted content because you only receive content from users you explicitly ask to receive it from. You aren't connected to the firehose when you federate with their instance.