I have not seen this before, but it is similar to other strategies that are used in runs. There is something called fast 4-2, and it's difficult to explain why it is faster than other ways of completing the level, but a clip like this is used. It is not used to actually travel faster, it is used to push Mario to the right of the screen in order to abuse the way the warp pipes load exit screens.
I believe one of these clips is also used to skip a small section in minus world or some other smb1 category/variant. I have never actually seen ground clipping though.
Edit: these clips used to be considered TAS only and not humanly viable. The inputs to make them work are very specific and quite tricky. I think that the next world record will probably include the 4-2 clip (if it isn't in the current one, can't remember) purely because there is not a lot of time left to save.
I'm going to do some more research and write up a post on this.
One thing I would like to mention for the moment is that people do find these strategies by themselves. Yes, there is also a collaborative effort across different games to pool together the best strategies, but there are some people who find a lot of stuff by themselves and then share that information. I think everyone who is capable of playing Mario Kart is also capable of finding a skip by themselves if they keep attempting different strategies and learning how the game mechanics work. AI does not glitch hunt, at all, but any human can.
Edit: I would also point a couple of other things out. The strategies used by fighter pilots do not come from the individual either. These techniques have been pooled together over decades. If you consider this, you could argue that the individual is actually more important in MK64 in relation to finding skips and whatnot.
I think another point to consider is that AI has a natural advantage over humans in any kind of combat situation. AI has a completely different idea of what risk management is when it comes to something that is life threatening. This exercise was a simulation, but the experience and training of a fighter pilot is always going to be based around staying alive as the most important priority, and that is still a factor even in this exercise.