I disagree that you don’t own it. Just because a business writes something into its terms and conditions, that doesn’t mean it is legitimate. The user behind the account has a stronger claim to the value of the account than the website - the user was the one who created the value, not the website. The website created the platform and then the marketplace, but the users are the ones who impart the value.
If the username is just a username and not being sold, then there isn’t really anything actionable, but because X are looking to sell it for significant value then it is actionable, and the user has the stronger claim.
This would be like a bank claiming all the money in your savings account because you haven’t made any deposits or withdrawals recently.