I think you're both right. I went to my first PMTS camp this year and have now read the new book. It took me some mental effort to square what Harald and the other coaches were saying about counterbalancing with the way I'd beeing thinking about my skiing. Previously my understanding of upper body movements came mostly from Ron Le Master's "The Skier's Edge".
I don't think either of you will disagree that in order to have your skis hold their edges, your centre of mass must be further towards the outside of the turn than it would be if you simply banked your weight into each turn. Harald calls the movement necessary to do this "counterbalancing". In PMTS, students are coached to make these movements by crunching their lateral obliques, or by reaching down with the outside hand to touch the boots.
I found this approach unusual. The range of motion you can get from your lateral obliques is actually quite small, and it happens reltaively high up compared to your knees or hips. Also, especially if you are a bit chubby, or female, a good deal of your mass is below your lateral obliques and can't be moved by them. You can see that for yourself if you stand, hold your hips and kness straight and try to bend as far as possible from side to side. I think Ron Le Master says this directly in his book.
For this reason, all the other ski coaching I've come across focuses on "angulation" at either the knees or hips. By rotating your knees inwards and bending them, you can move your CM appropriately. Similarly, by facing outside the turn and bending your hips forwards, you do something similar. Different people use knee and hip angulation differently because they have different degrees of joint flexibility and different distributions of weight.
As BigE has noticed, though, when the PMTS coaches demonstrate counterbalancing they show a good deal of angulation, at both knees and hips. This doesn't entirely jibe with all the talk about lateral oblique crunches, although I'm sure that if you pointed it out to them they would agree that these movements are necessary. I'm not sure whether knees and hip angulation are meant to result from tipping of the inside foot as the foot reaches the limit of its range of motion, or whether they're meant to result from the movement of the lateral obliques as they reach their limit.
I suppose it doesn't matter, and I suppose its consistent with the PMTS approach of specifying only simple movements and letting the others emerge as a consequence, but I'm not sure the simplification in this case was the best possible. Angulation is all mixed up in many skiers minds with old-fashioned and often incorrect ideas about counter-rotation, so there's no doubt some simplification was necessary.