That's the theory. One wonders what secrets the Sahara holds to be discovered, that may relate to a civ which carved the Sphinx originally. There's grist for a great novel in that, somewhere. I have one related to al Shamiya Desert, perhaps I should do one for the Sahara?
I don't know what the theory is. That is my hypothesis, which I came up with before any of these Egyptologists bothered to mention the obvious facts.