Yes but the default state is that you have copyright over your posts/comments, and by sending them to your Lemmy server you are giving them some license to at least distribute the content to others (most services specify what license you are giving them in the ToS, which is where they would say that you are licensing them to sell you shit to AI companies). In theory by specifying the CC-SA-NC license or whatever that should be the license unless your Lemmy instance has some ToS terms that specifically say you’re granting additional privileges to someone by posting.

Whether AI companies actually care (they don’t) is a different story, but if eventually they actually have to follow copyright laws like everyone else then it could matter.