He was convicted of embezzlement, forgery, and fraud. His entire career is that of finding ways of making money by unusual means and he hit the jackpot with this Alien nonsense.
The very fact that his made up evidence (misinterpretation of real Archaeology in favor of fantasy) points to an Alien technology similar to our own today, is proof in itself that it is nothing but nonsense. No civilization of our technological level of progress is capable of interstellar travel. So if we were visited by Extraterrestrials in the past or in the present, they wouldn't be using airplanes or light bulbs. There are technological projections into the future that are based on actual research, like that done by Ray Kurzweil or Michio Kaku, for instance, that illuminates, to some degree, what our future holds, based on current cutting edge achievements and trends. They demonstrate, beyond reasonable doubt, that by the time we are capable of practical interstellar travel, we would have no need for airplanes or light bulbs. It shows that these are nothing but fantasies of contemporary human beings. If this "theory" was developed during the Victorian Age, the Aliens would have been using steam engine locomotives. I don't believe that there was any "amazing" lost knowledge, there's no proof of that. There were isolated cases, particularly in the Ancient world, of scientific discoveries and engineering knowledge that was completely lost for over a thousand years during the Dark Ages and had to be reinvented during the Renaissance, but that's about it.
But at the end, there's no evidence at all, for any past Alien visitation. Though it is perfectly possible that we were visited or are being visited today, there's just no proof at all. It doesn't make any theoretical sense either, any Alien race capable of interstellar travel would have such incredible technological capabilities that if they wanted to help us, they would give us almost infinite power source, like Fusion Power Reactors, or if they wanted to destroy us, they could do it in an instant, or if they wanted to remain unnoticed, they could easily do that as well. It just makes no sense at all, from any perspective, that they would provide light bulbs and build a few runways in South America and maybe build a couple of pyramids in Egypt.