This all started when dataminers found on the eShop servers a Twilight Princess icon with
no Wii logo banners like for Wii digital titles -- this is what you would see on the Wii U OS if you had the game either in disc form or digitally. Because it had no Wii logo banners on the top or bottom of the icon, it was very suspicious.
Then about a day later, some people investigated Nintendo's title update servers using the NUS Downloader application which "downloads" whatever title updates Nintendo has newly added to their servers. Within this list of title updates it included new VC titles, new Wii digital titles,
and two Wii U games. One of those Wii U games was Twilight Princess HD, verbatim, word for word.
You can see the list
here which includes everything up to now. Just search "Twilight Princess HD" and you'll find it with its unique Title ID.
And then the damning part of it is the new Link render which was shown in one of the Investor's/Shareholder's Meeting slides. It is the
exact pose of one of the first artworks of Link for TP albeit in a more colorful way and him holding the Master Sword instead -- but very clearly is attempting to be an updated render much in the same way WWHD received an updated render for Link reflecting the new lighting changes WWHD received.