I don't think that's strictly true, if the rotation is giving the same length of cut, then the heads would seem to have the same drag. I believe Ashby's explanation for the better penetration is the splitting of tissue caused by the rotation, causing less drag on the rest of the shaft. e.g. the single bevel rotates, cracking the bone, causing a big gap for the rest of the arrow.
That being said, i don't think cut length is the only factor in increasing the chance of damaging arteries, liver, etc. At any given spot, the two blade is flat, and could be passing right by an artery. Hard to imagine a multi blade head doing that. I have no basis for this other than what I'm picturing in my head.