The ownership aspect of web3 doesn't fix the problem you outline, sure, but it isn't meant to. Ownership of something isn't proof of uniqueness, it doesn't stop copying, and it would have no impact on what user's are able to do with other user's content. That's the same as ownership in the real world - the fact you own a Picasso doesn't stop me copying it, or photographing it, or telling people it's really by me. Ownership is about control of how legitimate third parties interact with your things, not how people copy them, illegally or otherwise. The control over copying comes from the legal mechanisms we attach to ownership, not the ownership itself. You can sue me for copying something, or report me to the police. The same would be true for web3 (if the courts choose to recognise that aspect of it all).

The benefit of web3 is about users being able to control how their content is published by web3 websites. Reddit3 would not be able to claim ownership of user's creations. Like I said, I don't know if it's necessary or even that useful for users to have that control, but people definitely complain about it a bit. Maybe it would be.