I was curious about the X symbol so I looked around and found that X designations are part of the Mission Design Series (MDS) designation system. Mostly used for black ops projects. X-planes were what we would see. But I'm pretty sure X has become the symbol meaning black projects, in general. I believe Musk is enamored by these black operations and has taken X as a cliche for his own endeavors. The logo is so stupid and childish, it may be a joke or it may mean Twitter is going through a radical change. You don't just change logos for fun.