I don't know how it actually works, but I would guess it is something like this:

For the promoted Tweets, advertisers have specific criteria. So if an account doesn't meet the criteria of any advertisers, then that's not an account that gets "monetized".

Accounts with bots do have recent logins, accounts with bots do have locations, accounts with bots do have keywords in what is posted, etc.

But yes -- the viewing of tweets, where the promoted ads are shown, is probably minimal with those accounts with bots, and the advertisers probably know very well to avoid paying to those accounts.

So a bot account might not be in their 229M "monetizable" count, even though it persists on their platform. And thus wouldn't be something they cheated on because they only report monetizable users. That's probably a contributing reason why they aren't more aggressive about bots, ... they don't exist, looking through only a revenue lens.