> Seems like a peer that just pops up on a random address, spams out a bunch of follow requests referencing actor profiles containing crypto pump-and-dump scams, then vanishes would be just as much of a nuisance to Mastodon as an equivalent SMTP-based spammer.

I'm not thats anywhere near the disruptiveness of SMTP. As a private person, I could simply have the server not notify me of any follows and it wouldn't affect my own feed at all.