He's not giving any details or specifics, so there's really no point to this rumor other than attention seeking. By the time we know if this rumor actually ends up being true, it'll also be completely irrelevant. I guess you could say if BotW2 is confirmed for 2021 then that means he's also right about the existence of MK9, but that's a stretch.
Was definitely expecting BotW2, but if that's not ready yet and this rumor turns out to be true I don't know what to expect. Obviously won't be another Mario Kart, they aren't gonna release two Mario Karts for one system, especially not when it would just kill off sales from their best evergreen title and best selling game on the system by far. All I can think of is Diddy Kong Racing 2. But I think more likely this rumor is false.
In same breath you acknowledge BotW2 exist, yet say another Mario Kart is an obvious no go. Well, only time will tell!