> The 'when' is inconsequential, and is not required to know 'who'
I don't understand that comment. "When" and "who" are intimately connected. If that construction was done 50,000 years earlier, it obviously wasn't done by the Dynastics; it would have to have been done by whoever was around 50,000 years ago. Likewise, if the traditional Old Kingdom dynastics built the stuff, then the "when" is indeed "consequential" to the "who". In fact, it's possible that the "when" is more relevant than the "who", depending on the original purpose of those pyramids.
Maybe I missed your point.
> In order to identify
> these mystery people, there is no other way than
> to find archeological evidence in the ground.
Well, those monolithic structures strike me as some pretty awesome "archeological evidence in the ground". Depending on how old they are, we might not ever find evidence of any of their personal artifacts of the culture other than the construction they left behind since such artifacts are generally less durable and more easily destroyed, repurposed, or resorbed back into the environment.
Why you don't consider those massive durable monument artifacts to be "proof" of the civilization that built it is anyone's guess. It's certainly more definitive than any piece of cloth, bone or pottery that might have been deposited there by any number of adaption cultlures that occupied that infrastructure over the millennia. It's certainly more telling of their technical capabilities than the symbolic paintings in their later tombs, or in papyri that were scribed centuries after the fact.
How can any of us ever know, when all we can do is think?