I'd also add that the control system and general environment in terms of what is possible within Mario Kart favours humans. If the objective was to create an AI that could fly a plane and shoot targets, or simply drive around a track and finish a race in Mario Kart, I'd say Mario Kart would be easier.
An AI is much more capable when it comes to reaction times and decision making. I do not know what kind of controls were used in the flight simulation, but I'm willing to bet it was more advanced than an N64 controller. The flight sim probably did not run at 30FPS either. Whilst both of these things suggest that it would be harder to create a basic flying AI than an MK64 one, the opposite is true when the question is about high level performance. There is not a large amount of tactical decision making in Mario Kart, and the inputs are fairly simple. In other words, the two largest advantages an AI may have over a human are barely applicable in Mario Kart speedrunning.
Just a couple of points that I feel need to be said:
People don't discover the long range strategies unassisted either. Once one person comes up with a breakthrough that information is available to everyone else. Everyone else copies and optimises it. I think you're suggesting pitting one AI against all players, rather than one AI system versus the realistic performance of one human. I wouldn't be surprised if an AI could outperform the entirety of humanity starting from scratch with zero help anyway, but that is a different test.
One would reasonably expect significantly more resources and motivation to apply to a military application like fighter pilot training (and making fighter pilots necessarily redundant) than to playing Mario Kart. When I say 'limited resources and motivation' I mean relative to a research goal that's actually useful.