I'm not one of those people who thinks this is an indication that the Wii U is comparable power wise with the PS4 and XB1.
I'm also not one of those people who dismisses this as completely insignificant because of the 'cartoon' graphics, because that is equally naive.
Comparisons like GT5/6 and Wipeout aren't particularly valid because comparing their graphical style with Mario Kart is futile and they don't run a standard 1920/1080p 60fps (if we are going to picky).
I don't know what other games on the Xbox360, PS3 or Wii U have reached that mark but there aren't many and the only cart racer that has as far as I'm aware is Sonic All Star Racing on the PC.
So IF it's true it IS interesting and can't so easily be dismissed, that is if you actually care about this kind of thing. At the end of the day, however, MK's fortunes are not going to turn on it's resolution.