Fediverse

It's the same game as email. An arms race between spam detection, and spam detector evasion. The goal isn't to get all the bots with it, but to clear out the low hanging fruit.

In your case, if another server noticed a large number of accounts working in lockstep, then it's fairly obvious bot-like behaviour. If their home server also noticed the pattern and reports it (lowers the users trust rating) then it wont be dinged harshly. If it reports all is fine, then it's also assumed the instance might be involved.

If you control the instance, then you can make it lie, but this downgrades the instance's score. If it's someone else's, then there is incentive not to become a bot farm, or at least be honest in how it reports to the rest.

This is basically what happens with email. It's FAR from perfect, but a lot better than nothing. I believe 99+% of all emails sent are spam. Almost all get blocked. The spammers have to work to get them through.