It's not, look at some of the comments on the link. It's impossible to do this with the pokemon engine.
How would they know anything about the pokemon engine?
All this hack is doing is making a backsprite for the already present pokemon "ghost", replacing the trainers with the already present gravestone tile, making the trainer's front sprite into a pokemon, making text boxes, changing words, replacing the player's sprite with the already present "elder" sprite, slowing the walk speed, having an invisible pokemon (you), showing enlarged frontsprites, and calling screen transitions.
Seems within the engine to me.