Ehh....to be fair, if someone wanted to make an issue of it and force the removal of their posts that clause would never survive a legal challenge. Fundamentally it can't, as section 230 of the CDA comes into play at that point. In short, aside from the fact that posts made by a user still are, per copyright law, their intellectual property, the purpose of 230 is to confirm that the site can't usually get in shit for what posters post because its made clear that the author of a given post is the responsible for it as its their content (which is why if someone does something lawsuit worthy on a forum its that user you take to court not the owner of the site) If the poster was required to turn over rights and ownership of a post they would also be turning over any liability over it as well
In other words, if the site was to demand they own the rights to what you post, 230 would by definition no longer apply as a protection for anything a user posted and would inevitably lead to some kind of legal problem for the site owner sooner or later. The site owner can't have it both ways. Either they have 230 immunity and the authors are responsible and own what they post, or the site owner does and assumes the liability that comes along with it. Since posters automatically own the copyright to everything they post, resetera doesn't have a leg to stand on if they tried to enforce that moronic clause. If they were ever stupid enough to try to
That said, you can absolutely have posts scrubbed if you really want to go through the hassle of doing it and the paperwork and legal stuff that goes along with it. Copyright law is pretty clear on that. Not that many people have reason to do so or bother to do so, or want that hassle