I was in tune with this until he said that the retangular wing stalled at the root because the angle of attack there was the highest. Then I went into cerebral meltdown for a while.
There must be something else that also relates to the portion that you've quoted. And hopefully quoted in full and completely. As it shows in your post I can't help think that there's a reference to spanwise flow and vortex flow around the wing panels that is missing. I know that the vortices that are centered around the tips extend their effect well in on the wing. This is why sailplanes use very high aspect ratios be reduce the vortices and their effect on the airflow passing over the wing. It's not just a tip effect. So all I can think of is that there was some talk of vortex formation on the wing tips and how it affects he local angle of attack along the wing panel. And it was assumed in the portion you provided that you already had that in mind and that it was just a given that you'd understand about the rectangular wing having a spanwise reduction in EFFECTIVE angle of attack.
Do I win a prize?