Hi Greg,
I was at Mathieson's presentation on this at the Egyptology Scotland lecture at the Burrell collection a few weeks back. Mathieson has carried out high tech geophysics of the whole area around the Step complex over several years now and has amazing pictures that show just what's under the sand. Over the last couple of years he complete the blanket coverage and now has data right around the complex.
His comments are speculative, but based on hard data and hard logic - he has analyzed the excavation history of the area, the architectural styles for the relevant periods, and all of the geophysics data and come to a proposal that is certainly possible.
Also, don't forget that he needs the funds to be forthcoming to pay for this - so he needs the publicity - he finances the Saqqara Mapping Project himself and it costs a LOT - thousands a week in wages for his staff etc etc.
He was fund raising at the Burrell.
Dave Light.