I'm sure Twitter knows which are the bot accounts and is surely excluding them from their model training. Twitter bots aren't a new phenomenon after all.
Highly doubtful seeing as my 14 year old twitter account got caught in a recent bot ban wave with no means of contacting a human for recovery.
There is bots everywhere, it has nothing to do with the platform, it has to do with attackers having an incentive to do mass account farming, no platform is secure against it.
I don't think Twitter/X know for sure who the bots are, since Elon has been pretty vocal about trying to stop them for ages, yet I still get lots of spam DMs (as do others with far fewer followers/reach).
Even if 95% of the spam gets actively reported and dealt with, that still leaves a ton of nonsense on the platform, getting fed into the LLM. And spam has only gotten worse over the years, as the barrier to entry has lowered and lowered.