I'm sure someone originally "owned" X. AT&T? Oldsters (well older than me!) will know. They, and then the companies who merged with them or bought them out, would probably own the logo trademark.
The logos are similar enough, and they are both in the realm of "computers", therefore there may be issues. But if no one really cares about the X logo anymore, whoever owns it should sell it to Elon and make mega-bank! If this benefits the community (Xorg) somehow, then bonus.