This has nothing to do with the Wii U being hacked, at least not in any sense that anyone here would care about. It appears this guy was able to accidentally access an admin account of their Miiverse Wii social network that seemed to let him moderate posts and possibly users, and that Nintendo shut it down before he revealed how (which probably spared them having their first day being dominated by news of what 4chan did with those abilities).
He didn't "hack" anything (he just happened to press 'x' while hovering over the exit button), and it wasn't hacking the Wii itself in any sense that you might refer to the hacking of a console.