The creator is the automatic copyright owner, or in some cases their employer. Copyright is automatic through international treaties like the Berne convention. The Berne convention is from the 19th century and was created by the authoritarian european empires of the time. The US joined only in 1989. I think your question shows that the idea has not fully taken hold of the public consciousness. Automatic copyright is now the global norm. (I always wonder how much its better copyright laws helped the US copyright industry to become globally dominant.)

Very short and/or simple texts are not copyrighted. IE they are public domain.

Adding a license statement gives others the right to use these posts accordingly. It only serves to give away rights but is not necessary to retain them. The real tricky question is the status of the other posts. I’d guess most jurisdictions have something like the concept of an implied license. Given how fanatical some lemmy users are on intellectual property, not having it in writing is really asking for trouble, though.

What such a license means for AI training is hard to say at this point. The right-wing tradition of EU copyright law gives owners much power. They can use a machine-readable opt-out. Whether such a notice qualifies is questionable. However, there is no standard for such a machine-readable opt-out, so who knows?

US copyright has a more left-wing tradition and is constitutionally limited to certain purposes. It’s unlikely that such a notice has any effect.