Evidently the M&T joints were important, not just to the longevity of the structure but to do what M&T joints are intended to do - pull and set the pieces together during construction.

I reckon so.
And what they created in the outer circle was a load of uprights all connected with a ring beam. It enables any local strain to be redistributed and shared right round the circle. If one upright wanted to fall down, the others wouldn't let it.