Musk probably drew it himself

He took it from a tweet https://x.com/SawyerMerritt/status/1683310992493608960

Wasn't it basically the X11 window system logo?

It's an X from the blackboard bold range in Unicode.

It's confusingly similar (by the "average person" metric( but I don't think it's identical

The average person has absolutely never heard of X11.

The average person has absolutely no need to know what X11 is to compare the visual appearance of 2 images.

U+1D54F, MATHEMATICAL DOUBLE-STRUCK CAPITAL X

https://apps.timwhitlock.info/unicode/inspect?s=%F0%9D%95%8F

$ \mathbb{X} $

I don't think he'd be able to come up with this.

Musk probably came across the shape as a child and drew it on a napkin to carry it with him, like he did with the Cybertruck design after "beating" Car Designer on either the Apple II or PC