The trailer was probably recorded a) on a Windows version of the game made for debugging purposes (there is proof that Nintendo has done this in the past, you can take a Pikmin 1 GameCube ISO and they actually left the Windows .exe of the game there and it kind of works with some tweaking, I have tested it myself) or b) it was actually recorded on a Wii U, at 1080p.It pains me to think Aonuma has access to some 4k 60fps version of this and we'll have to wait maybe 5 years to play a 1080p remake on future hardware. I wish and pray with all my might they will announce new hardware at this E3 to play this Zelda on, or at least some kind of Wii U upgrade. I'm going to be one sorry panda if I have to play this at 720/30fps.
About the b) option: "but the Wii U can't handle that at such resolution!!!1!!!"
Yes you are correct. Not at 30 FPS anyway But nothing is preventing them from setting the Wii U to render the video frame-by-frame, even if it's actually running at 5 FPS or something, when you put all the frames together and get some audio in there, there you have a 1080p trailer. Or just take shots of the trailer at 4k on the Wii U, it's probably running at 1 FPS but it is running and that's enough to take a picture.