It's apparently a glitch in the code that was recently found, it requires a rather convoluted set of steps. But, I'd call it an exploit, you don't fight Bowser, you don't even go to world 8, basically skips a massive chunk of the game and goes straight to the credits.
I had a feeling that was the case. Can't they just set records the legit way. It's like running a marathon then at some point hop in a car and drive to the finish line.
Well, technically it is legit, the code was not altered, there were no cheats entered or anything like that, this dude just figured out what you had to do to trigger the glitch...and it only took like 24 years to figure it out. And you know what, I'm wrong, the world record is actually held by the man who originally found the glitch jw356 who clocked it in at 5 minutes, though it was never made official (he also did it on emulator, so this was a first on console).