The tens of thousands of dishes carved from stone as hard as quartz and corundum give pause. The ones found in tombs could have been made any time earlier than the tombs they were found in. But anyway I think most are officially pre-dynastic. Certainly the suggested method of manufacture is implausible on its face.

Suggestions that Egyptians melted and molded granite are obviously wrong, because the grain size seen in their products is achievable only with centuries- or millennia-long undisturbed cooling, possible only in nature.

Personally I think Hancock et. al. go too far, jumping to the lost civilization theory. However I also think the egyptologists are too conservative. The first thing anyone notices when they look at the Great Sphinx is that the head is too small. So then you look into it and find the egyptologists don't have any evidence it was built by Khafre it starts you asking questions, like did Khafre recarve the head in his own image?

Lost civilizations, as such, are entirely plausible. Anything older than the Pharaohs might be knowable today only from stone and ceramic work, and there is no inherent need for a civilization to devote much effort to those. Anyway if they did, a later one might be able claim credit for the remains. (Cf. the copious pre-dynastic stone dishware.)

We now know Amazonians were domesticating trees at least 10kya, known today only from their major earthworks and from genetic traces in plant genomes.

Professionals can discuss any of that only in the context of evidence found and revealed, which might not be forthcoming for any number of reasons, past and present.

So while I enjoy that theory, I feel like it's pretty much a more gentle form of the insane mud flooding theories that all buildings are older than we think. We have plenty of contemporary texts confirming construction of these buildings. Nobody else seriously challenges their legitimacy in history other than recent conspiracy theorists.

Proponents of this theory consider the Great Sphinx as one the best bits of evidence for it precisely because there is not a a single contemporary inscription attributing its construction to Egypt/Khafre.

Yes, you don't put up a stele claiming credit for repairs to something you built in the first place.

Circumstantial evidence for tunnels found in and under the Sphinx is intriguing, and not prima facie implausible. (The Egyptians did love tunneling!) If true, one wonders why authorities continue keeping them secret. Perhaps there will be a big reveal someday in a way calculated to benefit someone's reputation, but it may be that any such reveal could only ever embarrass.

The big, empty, precisely-machined boxes in Saqqara with painfully crude exterior surface decorations are another big mystery. Again, one wonders what more authorities know about them than is yet published. All we may be certain of is that the official line is bollocks.