Mark:
Squares are rectangles.
You and I see very different things in those Myers Park greens (just look at 17 alone). There are certainly a couple that are way closer to circles than they are to any kind of rounded off square. But perhaps we're splitting hairs here.
The point I was trying to make is that Ross built greens of all shapes.
He built greens of all shapes and sizes. He was building greens over a nearly 50 year design career, through a few different eras of design trends. He built courses during the "geometric" or "victorian" era of design, and he built courses during the golden age and beyond. He built courses that had sand greens. Many of those were nearly perfect circles.
If there's one thing you can say about Ross is that his career, and his courses, defy definition.
So to say he didn't build circular greens is a fallacy.
To answer your original question, early on the green was for the most part a continuation of the fairway, just mown at a lower height. If you continue the fairway lines for the length of the green, you get the squared off shape you're discussing.
When greens started being built up, the became more of a separate part of the course, its own entity and no longer merely an extension of the fairway. Around that time, the idea of replicating nature had fully taken hold. The curve is way more natural than the straight line.
Sven