The only other possibility is that the Wii U AC that was most likely moved to Switch was an enhanced version of New Leaf and had this feature in it. They realized it would be too far off so they downported the feature into the 3DS version. That's my theory.
I bet Splatoon 2 is the Black Friday pack in. They already had Wii U Mario Kart bundles and it is just a port. Splatoon 2 is atleast a new game in their hot new franchise.
Well I think they should/could do a $50 price drop and bundle in MK8 Deluxe at the same time. Another advantage MK8D has it that is shows off single Joycon play.