to be totally honest, I'm pretty sure whoever wrote this has no idea what the question is discussing.
keep in mind, that A) the miscast is still applied wholly to the opponent.
B) the opponent has no power pool, the question has nothing to do with WHOS power pool the dice are taken from, only weather they switch to the dispell pool. I'm sure that whoever wrote this assumed one of 2 things.
either A) the the dice are removed from the sufferers next power pool(I haven't seen it played this way)
or B) they didn't care, as the question just asked whether the player could switch the word dispell for the word power.
This is exactly why I typically don't pay too much attention to the GW FAQs, as the FAQ portion is useless since theres absolutely no context to any of the answers(id rather know WHY the answer is what it is, not THAT the answer is what it is).