Yes, creating blocks still takes lots of computing power, but isolating someone from the network means it's no longer a race; if it takes a day to make six block, then after a day, the victim (which may be an automated process) will think they have six confirmations. It does still cost 50BTC of computing power per confirmation on the forked part of the chain, since those blocks won't be built on. This is a good enough deterrent in most cases, but it's not as strong a defense as requiring 51% is. Ordinarily this attack wouldn't work at all, even if you spent hashing power on it, because during that time the target would almost certainly hear about the main chain, which is longer.

Actually, there might already be some mechanisms in place to notice and warn if the observed block generation rate drops precipitously, which this attack would cause. I'm not sure about that; if there aren't, there should be.