Unfortunately for reddit, there are many backups of that data that aren't held by reddit.
If some new AI was trained on those backups, could Reddit take the creators to court for using that data without a license?
I've always thought it fascinating that people claim authorship or release things under a license without disclosing who they are. I imagine something licensed by Donald Duck is not actually usable under that license.
If it worked like that anyone could re-release anyone else's things under a different license. I mean, which Donald is the real author? How are you going to hunt the duck for violations? How do I prove I'm the real duck?