He literally got removed from Paypal, "his first company" for trying to push the x.com thing there. That's why he owns the domain. It was stupid then and he held a grudge ever since which is why he's pushing it everywhere else since.
Ultimately, Levchin gathered several other key people in the company, and went to the investors threatening to quit (and destroy the company) unless they brought back Thiel as CEO. The board sided with Levchin, Theil returned and Musk was out of day-to-day operations.
If you’re interested in the PayPal story, I highly recommend Jimmy Soni’s book, The Founders. https://www.amazon.com/Founders-Paypal-Entrepreneurs-Shaped-...
I mean, while nothing about the web was particularly mature at the time, the idea that Windows was more mature than Linux for it is… peculiar, from the point of view of someone who was messing around with web programming at the time. Note how much difficulty Microsoft had migrating Hotmail to Windows.