Ironically, the site has been down for a few days now. Also, X Corp holds the Twitter trademark.
Even if the startup wins the fight against X Corp's claim to the trademarks, it then means the startup has to convince a large number of people to abandon x.com or the other systems that have sprung up. The network effects cards are stacked against.
The whole approach feels amateurish and non-strategic, BUT if they pull it off, it would be pretty astonishing so I wish them luck.