See, he gets real cagey with what he considers "an advanced civilization". The implication is like a Roman tech group, somehow living in isolation in some now inaccessible place (island that sunk into the sea) that could circumnavigate the globe, peacefully interact with numerous stone age tribes to guide their development, but somehow leave no tools, pots, ships, or carvings behind, no structures, no bases, mines, ports, or bridges, no real linguistic connections, and yeah, it took 1000s of years for their guidance on domestication of animals and plants, development of technology, and cultural growth to have an affect. Not once did these guys import seeds, animals, or material (knowingly or unknowingly) in ways we can not account for with natural migration today, they lost no ships in any areas where everyone else did, and they built no structures in prime areas where everyone else did.
The idea is romantic, sure, but not plausible. More likely would be civilizations from 80K years ago wiped out and starting over from the supervolcano, but even then they cant have been mining much because evidence of that lasts practically forever.
