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.
Honestly, it's pretty safe bet. ARMS was made by the same team and that released in 2017. It would be time for Mario Kart assume they went to work on that after ARMS. Also, the hint is dumb because you might as well just say what you mean (possibly could be guessing as well. If Mario Kart 9 doesn't happen, could be referring to anything and claim "I never said it was Mario Kart).
Honestly, if the Switch were to get another major first-party racing title, I'd rather it be F-Zero. Nintendo has been sitting on that IP for far too long. The last entry in the series was over 15 years ago on the GBA, and the last console entry was a year before that. I was 23 when GX came out. Now I'm almost 40. Somebody that was a toddler when the last F-Zero game was released would be like a junior or senior in high school now. Younger gamers may only know of F-Zero through Mario Kart 8 and Smash. Nintendo shouldn't let their IPs remain inactive for so long, especially when there's clear demand for them.
Nintendo has been bad with a lot of their series. Why they haven't made an F-Zero is beyond me as the demand is there. It's been gone so long that you'd likely have people who have never played it before try it out. It also gives them a different title. 30 Player online multiplayer. Assuming it works, it's a great sell over more Mario Kart.
Visit my site for more
Known as Smashchu in a former life