Around the 4-minute mark he says he's running the games at 480p. This is the original resolution of the GameCube, Wii and Wii U vWii mode. The 'HD' is that the Wii U can output to HDMI (GC and Wii needed a third-party adapter) and has a fairly basic upscaler to 720p or 1080p (480p is also an option). This isn't increasing the resolution the games render at - you would need to use Dolphin on a computer for that - the closest you will get to HD GC on Wii U is setting it to 480p and using an upscaling HDMI cable that does a better job than the Wii U or your TV (e.g. Thor, Seiki uVision, Marseille mCable or mClassic).