First and foremost, I suggest you try a high pivot bike if you haven't already.
That aside, I think you make a good point that the inertia of a rear wheel when rear suspension is rebounding can contribute to the forces transferred to the rider, and it can do this more for high pivot bikes going over square edge bumps. However, what I think matters more is the alignment of the direction of forces from bumps with the direction of rear wheel travel. A good way to understand is by reducing things to an (absurd) extreme. Imagine a bike where the rear axle path is a straight line tilted 45 degree from vertical and towards the front of the bike. Now imagine a bump on the trail causing a force transferred to the rear wheel with a direction that is angled 45 degrees from vertical towards the back of the bike. Because the direction of the force and the direction of travel of the rear wheel are at 90 degrees to one another, there will be no rear wheel travel from this impact. Therefore, a maximum of force will be transferred to the rider (like riding a hardtail) even though the force transferred to the rider because of the rebounding inertia of the rear wheel is minimized. The rearward tilting of the axle path of bikes like the Highlander generally increases the alignment of bump forces with axle path direction, and generally decreases forces transferred to the rider when plowing through rough stuff. Note that this is my current understanding, and that I might be wrong because my understanding of these sort of things constantly evolves. However, I have tried a high pivot bike and they are pretty awesome in the rough.