There is no Sybil attack possible on the problem as stated. "A majority of CPU power" is a physical quantity which can't be Sybil attacked. Period.
True, but there are other "attacks". Such as calling up Chinese miners and convince them to do a certain thing.
This only works because Chinese miners have a majority of the CPU power. Otherwise you call them up all you want, but would accomplish nothing. You might as well call someone with an old USB miner stick.