They can have it both ways. Usernames can have value and Twitter can sell them and users can not sell their own accounts.
I looked into who owns the tweets and Twitter said users own their tweet but a us judge ruled that Twitter owns the tweets. I don’t think it’s reasonable to think you own a Twitter username and I think its reasonable for Twitter to delete your inactive account and release the username and sell it if they want. I don’t think you would win a legal battle and Twitter can update their policy to do whatever they need to do to remove your ownership if you had any.