You do have a weird point, looking at it frame by frame, there are around 2 frames which he appears to stand on nothing. I'm assuming, however, that this was a bug in the game with the hitbox of the platform right ahead of him. I'm surprised you noticed that in all fairness.

EDIT: To add further, I highly doubt this was a laggy scenario, as lag doesn't Usually effect player position, only player input, (These are what I have noticed, if someone experiences other stuff with lag, well, take their word for it.)

EDIT 2: Upon further inspection, it appears to be the platform behind him. Still unsure. I don't know what to think. It is weird that where he "Double Jumped" is in an area which is in the middle of a gap in two platforms. Thinking about it that way, if the hitboxes were extended, he'd practically be stuck... I'd like more input on this.

yeah it's most likely just the hitbox of the platform being larger than the actual platform itself

edit: additionally, the players hitbox comes into play too

bear in mind that it takes just a tiny amount of the player hitbox to register as "on top" of a platform, the character could have like a couple pixels on top of the platform and still be able to jump

He is not cheating in any way. This is just how the hitboxes in the game work.

roblox player hitbox sucks

nah it's good ur just thinking too hard, roblox hitbox is bad.

I think this is a result of being able to jump without fully landing on the ground. This can occur just by holding space - you'll notice that your character jumps without touching the ground. The WR just happens to do this at the edge of a platform (as indicated by the shadow) so the jump appears to be a double jump in this case. This isn't considered a glitch by itself, so the run is legitimate.

Lol vurse just freakin broke the game, he sucked