I don't have proper confirmation it cannot be accessed but I have seen the assets on the game files. Since it hasn't been announced and it's not part of the cheats (Which I can confirm almost completely) I'm afraid it's at least not officially in the game.
Since there are leftover files on there, it might be possible for someone to enable kart mode somehow.
Still, kart mode if not already secretly accesible in the game, will be added to the game very soon. Also the code is going open source in a few months apparently.
