Yeah, I'm pretty sure that doesn't make any sense--as I recall, he controls the werewolf by controlling his emotions, not the magic. Making it so that it works the way this implies, and that he's non-magical when he's not a werewolf, is fine although it ignores things like the fact that he still has a wolf's sense of smell as a human--except being a Slayer isn't really an on/off thing, so I don't know how Oz is supposed to help. Maybe they'll show a flashback where he somehow picked up some anti-magic item?
Anyway, all that aside, I will gladly accept any excuse to have Oz appear, no matter how illogical.
Also I really wish they hadn't named the big bad Twilight, because it keeps making me think that Edward visiting Sunnydale, getting mocked by Spike and then having his head ripped off by Buffy is going to actually happen any minute now. But no.