That's such bullshit, he said that he couldn't use the curse move against his rival then that means that he wouldn't have been able to beat his rival in professor oak's lab at the very beginning.
Ghost can use Curse against the actual enemy trainer. Not just against the Pokemon the trainer has.
So when he finishes killing bug trainer's pokemon with curse he KILLS the actual bug trainer with Ghost's curse move.
But you can't kill your rival until the last time you face him at the end of the game. But you can still use curse to kill his pokemon in matches previous to that.
Man if this mod is real it's pretty messed up. Especially the ending.