After lots of musing and visualizing, I think I understand this a little bit better. I believe it has to do with the "L" shaped cage that Jim mentions. Essentially, when shifting from larger to smaller cogs, this "L" shape will cause the jockey pulley to rotate up, but it will also cause it to rotate forwards, towards the front of the bike. So if it starts out directly underneath the freewheel axis, it will end up well ahead of that axis. The parallelogram motion contributes to this as well, albeit to a lesser degree. So the sprung upper pivot will rotate the entire derailleur towards the rear to compensate. But the issue with switching chainrings remains a problem with this design, making the SunTour still superior, IMHO.

I searched YouTube, hoping to find one of Kurt's ("headbadge") videos showing a vintage 600 derailleur shifting over a vintage freewheel, but didn't find any. When my mixte build reaches final assembly, I'll mount my Arabesque GS derailleur and my wide-range freewheel, and make my own video. Until I do, the above is just conjecture...