XNB files are not meant to hide your data, so I wouldn't count on them to do so. The file being binary and not plaintext is just a side effects and not the main goal of it.
For example, this article talks about a tool to extract PNG from XNB:
With that said, are you sure you really want to hide your XML files? If players want to cheat / mod let them have fun, and if its multiplayer game you should not put so much trust on client side anyway.