Seems like Xanatos is revealing that he knows a lot more then his character would hint at knowing.
I have to agree with what some have said right below today's page, Xanatos seems awfully Starscreamish this page. The fact he knows more about Addy's history is quite disturbing.
Though the hint about joining The Endless has me wondering if he's hinting at her to join them or maybe they might be one of the things that got in her way to the throne?