I don't know much about programming, but here's my guess.

DDD doesn't move faster in this game compared to KSSU, but whenever KIrby gets close to the finish line, it triggers a loading zone which teleports DDD to the end. When you fly to the top left corner at the S-shaped section near the run, and then move to the right (like what ProfanedGlitch did in his video), it triggers the loading zone and tricks the game into thinking you're about to cross the finish line, and so it teleports DDD there.

Simply let Dedede win because the IGT stops once he wins.

Just watch ProfanedGlitch's video

This might sound kinda stupid (it probably is) but I was thinking if a Challenge Stage category could be a thing. Timing could be done by either:

1. Measuring RTA from entering the first door of the first stage to entering the last door of the last stage, or
2. Adding up the IGTs of every stage

P.S. I'm sorry if this question has already been raised before.