It isn't, like even a little.

I've got a wordpress instance that I've federated into the fediverse and as soon as I did I went from 2-3 spam comments a week to over 50 a week. I've since set up a honeypot and that has dropped to zero. These probably aren't AI agents but they're certainly bots (humans can't see my honeypot unless they're browsing the raw HTML and afict my audience wouldn't do that)

There is absolutely nothing stopping your from spinning up a Masotodon, Lemmy, or any other AP federated platform and unleashing AI agents all across it other than your own conscience.