If you use xHamster's tool to create or link an account on another site, your messages are no longer just shared behind the scenes to set up the account. The new wording says the content of messages you send or receive — plus the usernames involved and message details — can be displayed on that other site for people to see. On an adult site, that is sensitive information, and once it appears somewhere else you may have little control over who sees it or how long it stays up.
“the operation of linked account features across those platforms, including messaging functionality, with the result that personal messages sent or received through your account, together with the associated usernames and message details, may be displayed on the third-party platform(s) and viewed there”
What you can do — Avoid using xHamster's one-click tool to create or link accounts on other platforms. If you already have a linked account, check that platform's privacy and message settings to see what is publicly visible, unlink the account if you can, and assume anything you send through a linked inbox could become visible to others.