They don't claim copyright, they ask (since this is a EULA and they don't tend to stand up very well in court) for a license to do the things you expect of a chat client with the stuff you post on the chat client (the big distinction being this doesn't restrict what you can do with it at all). It's a symptom of how broken copyright is that it's necessary. A really important part of that clause is "in connection with operating and providing the Service". While there's possibly a wide range of interpretations of exactly what that would mean (for example, could they incorporate any code you post on the platform into the code used for the server or client?), it means they can't do whatever they want with the content.