He seems to contradict himself a bit by saying there is ZERO couple when the bar is horizontal but also says that a panhard and watts always have some coupling but that it can be minimized.
Do you remember where you found this comment? I haven't dug into Milliken yet, but I did check a few other books this morning.
Herb Adam's comment applies to the OP's question: "a well-designed and well-developed live axle will beat a poorly designed independent rear suspension, even on rough roads. On smooth roads, is is usually difficult to see any advantage for an IRS." He also advocates a low rear roll center, which admittedly can't be as low as a SLA design.
After reading Herb Adams, Alan Stainforth, and Carroll Smith, none directly address our question. Carrol Smith actively hates solid axles! The common theme: None go into detail about an angled PHD, (I assume because it's just poor design) other than to say it increases the asymmetry inherent to a PHB.
My take: I think jacking effects are always attributed to Independent suspensions because they can be more severe and progressive, think swing axle. They also allow you to place the RCH almost anywhere, including very high, while it's hard to more a solid axle RCH outside a limited range.
Why I think they have weight jacking: We both agree a SA has a RCH that can be varied. The definition for RCH does not change for a front or rear suspension, so the function is the same. Smith: " ... is that point about which the sprung mass will roll under the influence of centrifugal force... the point through which the lateral forces from the tire's contact patches act upon the chassis. " Dynamic Load Transfer: " load transferred ... due to moments about the CofG or it's Roll Centers. " He then talks about weight jacking in the IFS section, which every author does, I think creating the common perception that it's only an IFS/IRS problem. Remember, authors are not perfect, contradicting themselves, especially if we take every sentence a gospel. " If the roll center is above ground, then the line of action between the tire contact patch and the Roll Center will be inclined upwards toward the vehicle centerline. ...the side force developed will have a vertical component which will lift or "jack" the unsprung mass."
So, a solid axle has a Roll Center, and it is above ground, and Physics don't change with suspension design, so it must have a vertical component. To prove this assumption, changing the RCH of a PHB changes the handling balance and roll force distribution. The only way it can do this if it takes load OFF of the springs. How does it transfer this load? From the chassis connection to the contact patch. Connect the dots and you have your inclined force line and jacking. Think of it as a J-bar, it doesn't matter that it's curved, the force is still transferred in a direct line from the two end connections. This is analogous to the contact patch and the PHB chassis mount. The actual load path may be up through the tire/wheel, to the axle/PHB axle mount, PHB to chassis, but the force vector is still a straight line. In your assumption with a flat PCH, where does the load go, it can't end at the axle side, because is has no connection to the ground. It has to pass down through the wheel and tire.
There, you officially got my longest post on any site, by a long shot !
Happy Easter, I'm going to Brunch !