When in reality they just went "oh hey, double struck mathematical operators? Cool", typed "&1D54F;"[1], scrolled through their font picker, and then sent the invoice.
Presumably, Elon Musk did it himself. Which isn't hard to believe considering his fascination for the letter "X" and that it is a common Unicode symbol.
The story doesn't tell if he paid himself to do that.
I think it's a joke on the claim that Twitter would become an 'everything app' when it became X. The software X11, being a windowing system, actually is a system for running 'every app', in a way.
Maybe they were joking comparing the logo flip with the way X flips the regular notion of client and server (you run the X server on your machine, and you connect the remote app as a client).
[1] https://www.fileformat.info/info/unicode/char/1D54F/index.ht...