When I was first trying to get all the secrets in the level, I found this by accident too! But I didn't really bother to explore too much out of bounds since I knew there wasn't really any point in backtracking. Its a pretty cool out of bounds glitch tho!
It's called a slam, it's done by pressing the slide button in mid air. Slamming and jumping repeatedly will increase the height you gain, and there's a tech known as slam storage where you slam next to a wall and before you hit the ground, you jump to prolong the slam, causing you to jump very high.
the thing that happened here is:he jumped very high so the previous room didnt load in quickly so he "thought" he spawned in the room but a bit higher than usual. this is called like, loadglitching or something idfk