It's legit, it's just a crazy glitch that requires extreme precision and spot-on timing of everything. To stress how crazy it is though, back around the turn of the century, I got a Game Shark for my Nintendo 64 and once decided to have some fun with Mario Kart 64 using it. I decided at one point to use a code that let you start each race on the final lap. That way you just rocket boost across the starting line and the race is over. In Time Trial mode, this translates as course times of one second. And yet these people can finish and individual lap in Luigi Raceway in less than half that time. Think about that: the world record lap times on Luigi Raceway are so fast that you can't even beat them by cheating with a Game Shark! That's just nuts!

Then you’ve gotten pretty good if you can do all those things consistently, because they’re not easy.

I also had (have, somewhere) the Nintendo Power Guide, so I tried most shortcuts back in the day. I would get most at some point one time, but never with any reliability. Note I was between 10 and 16 when I played it most.

The Wario Stadium one around the start/finish I got like once or twice in total, and never the return jump. I basically never got any of the ones that cut basically the entire course like the Frappe Snowland one. I did get the Rainbow Road jump fairly consistently, but that one’s one of the easier ones of the really big shortcuts I think (making it over there anyway, not necessarily the guardrail thing), and the DK Jungle Parkway occasionally. I also remember one on Royal Raceway at the big booster jump I think and skipping some walls in Bowser’s Castle if I’m right.

Pretty much ALL records are like that. The people who set them aren't just good, they LIVE the game. Twenty-four-seven. That's why they set them. Because the rest of us just aren't willing to loose our jobs, wives, girlfriends and sometimes... even our health for a record in a video game. 

It's actually quite inspirational their dedication to a cause.

I've never really got Mario Kart 64. The drift boost mechanic isn't intuitive for me on that one like double dash and later, I usually don't even bother drifting in mk64 whereas in later games I almost never stop. So I don't think I'd have a hope here.

Having played a lot of Mario Kart 64, I can't drift without doing the left-right-left-right motion for mini-boosts in more recent games although it's not required anymore.

Wow! It's just the opposite way around for me: I still have tons of trouble with the ramp shortcut in Royal Raceway to this day. That's like the toughest one in the game that you can find in the guide to me! The fact that the ramp also boosts your speed throws off my angling. I find it much easier to make the first wall jump in Wario Stadium, for example, precisely because there aren't any factors like that involved. I can pull off at least that first wall jump in Wario Stadium like 4 out of every 5 times. Jumping through the wall on D.K.'s Jungle Parkway is actually pretty tough for me too, but I can pull that one off more often since I know basically where to jump and basically how; I just find it tough to actually execute at top speed (which I've discovered is, in fact, necessary) given the precise angling required. Still, 1 out of every 25 or 30 times I can pull it off on all three laps and that's all you need to set Time Trial records.

I will agree with you though that fundamentally making the long jump in Rainbow Road is actually among the easier big shortcuts in the game. Once you know the tile you need to jump from, it's a piece of cake. It just looks fancy...which makes it even more fun to do.

Oh, the Frappe Snowland trick is super easy actually! Just turn around from the starting line and drive onto the wooden bridge, then turn around again. From there, just hop directly from the bridge onto the right-hand snow bank and drive out on the snow until Lakitu picks you up, being sure to pass the starting line before he does (i.e. don't drive out at too far of an angle). From there, he puts you down back on the wooden bridge. Just repeat that last step two more times; back up and hop directly onto the snow bank and drive out past the starting line until Lakitu picks you up. Each subsequent time, you skip a lap. Then on the final lap, just drive through the starting line. In Time Trial mode, I recommend using your mushrooms after you land on the snow bank to speed up the drive into Lakitu's waiting fishing lure. Just make sure that you hop DIRECTLY from the bridge to the snow bank on the right each time because if your wheels touch the driveway in-between, the trick won't work. After a few tries, you'll likely have mastered it and be able to consistently finish the race in under 50 seconds, if not under 40, including in Grand Prix mode.

The only times I've managed to like skip whole parts of Bowser's Castle in Mario Kart 64 was when I got hit with a shell that knocked me clear to a different part of the course...which of course ain't possible in Time Trial mode and were freak accidents anyway. Maybe you discovered a glitch that I didn't!

Some of the legitimate ones, yes!

You ain't alone because back in the day, I was the only person I knew of who ever figured out how to mini-turbo in Mario Kart 64. A couple other kids I met figured out how to power slide, but even after I explained mini-turbos, nobody else seemed able to master the art. This made besting the competition easy for me. It also made races against other people pretty dull and one-sided in general though.

I don't think it's all that difficult really myself. Just make sure you hold the directional stick opposite the direction you're turning after you've made the initial hop into the turn. That'll keep you from sliding too sharply and enable you to retain basic control of your kart. Of course from there you just jerk the directional stick left and right a couple times and release. Sliding itself has become so reflexive to me that I find it easier than trying to simply steer through turns. I feel like I have more control over the kart while sliding.

I learnt (or rather perfectified) drifting on Mario kart 64 after playing Mario kart 7, while it should be the other way around. :) I love drifting in Mario Kart 64, because its the most difficult among MK, I change my mind tough when facing Japanese players in recent Mario Kart games...