Except US trademark law doesn’t work like that and hence the case.

One of the core things a company needs to do to demonstrate it needs a trademark protected is that it’s actively using said trademark. If X has effectively abandoned Twitter, then it can’t really claim it still needs to protect the name.

What you’re describing is more the “Kleenex” effect where words become commonplace. In this case another company builds a platform that has “tweets.” There it’s even more important that the company with the trademark shows that they use the trademark. Here the owner has basically scrubbed the trademark from their existence hence why folks are making these claims.