We're sick of closed walled-garden monoliths like Reddit! Let's move to an open federated protocol where anyone can participate and the APIs can't be locked down!
...wait, not like that!
Yeah. This is what you signed up for when you joined the Fediverse, the ActivityPub protocol broadcasts your content to any other servers that ask for it. And just generally, that's how the Internet works. You're putting up a public billboard and expecting to be able to control who gets to look at it. That's not going to work. Even robots.txt is just a gentleman's agreement, it's not enforceable.
If you really want to prevent AI from training on your content with any degree of certainty you're probably looking for a private forum of some kind that's run by someone you trust.
I really don't see how it would be physically possible to do that and still allow the content to be publicly seen by other humans.