That was like one of the first things I noticed when we had our first look at the game. And it looks like they almost named it that from their Ask the Developer interview.
Super Mario Boards
Hint Toad
LuigiFan24
Everyone thought it'd be titled Mario Kart 9 as well, and they were technically correct lol 
Meta Knight
They were dissuaded from following through with the name because they'd receive over 1000 emails stating "Um actually Mario Kart Tour is Mario Kart 9"
Hint Toad
And also Mario Kart Live: Home Circuit would be their 10th game.