He didn't keep the x.com domain. He purchased it from PayPal in 2017. [1] Indeed at the time Musk tweeted Thanks PayPal for allowing me to buy back http://X.com! No plans right now, but it has great sentimental value to me. [2]
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[1] https://www.theverge.com/2017/7/10/15949862/elon-musk-x-com-...