Taking the leaky nature of the Mario Kart DLC thus far as evidence of it being primarily externally developed is a
massive stretch. Mario Kart Tour is an even more open book for datamining (those paying attention already know the next five Tour themes covering now to the end of May, and plenty of racers and courses releasing after that), so I don’t think it says much that BCP has also had a good amount of content leak thus far.
At the very least Yabuki seems very much involved with the BCP;
retweeting and liking tweets related to Wave 1-3 (nothing for 4 so far). Wouldn’t be surprised if Bamco were assisting like they have on every EPD 9 project since vanilla Mario Kart 8, but i’m sure the core work is being done internally.