Musk has owned the X.com domain since at least 2017. [1] And even that was buying it back from PayPal. X.com was an online bank co-founded by Musk in 1999 which was later on merged into PayPal. [2]

[1]: https://domaininvesting.com/elon-musk-acquires-x-com-domain/

[2]: https://en.wikipedia.org/wiki/X.com