It's not a ROM hack; it's a stand-alone game for the PC. Besides, if it was a D/P hack, I wouldn't be using 2D tiles :P
To be honest, I don't know a thing about hacking ROMs. I've played a few hacks, but I believe making a game with a game maker is less limited than hacking, as it's a lot easier to edit the core mechanics of the game, plus there's no space restrictions (in hacking, there's only space for 25 more Pokémon, giving a possible total of 386+25=411. I already have all 493 existing Pokémon, and am adding at least 100 more. Top that, hackers :P
Oh, and do you think we should have a game-making thread in Renaissance? That could be interesting...