You expect a journalist to field tech to scan continental shelves? His role is to call attention to unaddressed questions provoked by evidence. It is historian's and archaeologists' job to answer them.
Do I expect the man to go swimming around the ocean's depths? I have no expectation one way or the other. My only expectation is that, having not done such work, he doesn't claim to have the result of the work he didn't do. I assume GP was thinking similarly.
Look up his book Underworld
He speculates, badly, in a vacuum.