Musk probably thought calling it "X" made it sound edgy and cool. I bet it really annoys him every time it's called "the social media platform formerly known as Twitter."
Apparently he has been obsessed with the letter since before the PayPal days. He owned X.com before joining PayPal. It was even named X for a little bit. Before it went public the name was changed to PayPal. Because of course X is a terrible name for brand recognition and marketing. I think part of this just fuck you to all the guys who started PayPal
Oh and remember the only real thing Musk brought to PayPal. Was his daddy's money.