He didn't say he wanted to work on it. He implied that he was already working on it (in some capacity) by saying "I can't get into the specifics, but I do want to create a new and enjoyable Mario Kart that uses the unique functions of the Wii U. Anything beyond that I have to keep secret. Please bear with me for now!"
So he is obviously atleast in pre-production if not in full blown development when that quote was made back in November of last year. That much is clear and is exactly what I said in the other thread. Nintendo is working on it, nobody seems to doubt that except for you, so just let it go.