By the way, in case anyone reading doesn't know what we're on about, it's the bloke who wrote books saying we've been visited in the past by aliens who were responsible for various achievements such as the pyramids, the Nazca lines, early maps, and that certain ancient symbols, etchings, carvings etc depict these visitations. And therefore dismissed in a single sweep the inventive genius of a number of ancient cultures. Indeed, in insisting the Nazca lines were made by aliens, he referred to the Nazcans as "these primitive idiots".
For you to say "no one knows whether or not there could be some truth in his theories" is not simply bending over backwards to accommodate this liar, it's bending yourself into a pretzel. What we can do is look at the evidence and see that it points clearly to the inventiveness, endeavour, intelligence and resourcefulness of many ancient cultures. If you're essentially saying "Ah, but none of us were there, so how do you know these things weren't all done by aliens, eh?" that is a dead-end - it's good to be open-minded but not to the extent of allowing the possibility of outlandish 'theories' at the expense of dismissing our own achievements.