Important remark: H2LASO is a well-known challenge and has been completed before, but it's mostly doable due to one of the "skulls" (modifiers to make the game harder). That particular skull made you invisible if you activated your flashlight, which made large parts of the game much easier. Still hard as balls, but doable.
This particular challenge, however, was to complete the challenge without that particular skull, making it incredibly more difficult. I've seen clips of people trying it, and getting assblasted immediately when they entered a combat zone. And with the "no deaths" rule, that means you gotta start way from the beginning. On top of that, no saving/loading was allowed. The entire thing had to be completed in one sitting.
Completing it is an insane feat of gaming. Props to that guy. 20k well earned.