--and there is evidence to suggest that these secret machines have been developed for decades, if not longer, but that they may not be what we think they are--we believe it is more important to consider that it may not be alien technology that is being re-engineered.
If you think about the stories of the Vimanas, the lightning weapons, etc., then look at the megalithic evidence of machine marks, and then think about the "secret" knowledge supposedly held by secret societies, or the power of the priest/king class to perform magic/miracles it makes some sense.
There may/probably was a society/civilization that had high technology. Call it Atlantis for fun. As some have been proposing some cataclysmic event, or series of events, destroyed that civilization. Some of the survivors and their technology remained. But over the centuries that knowledge was lost or hidden away because there was no way to reproduce it or repair it. But what has remained of it is still in the hands of those secret societies, including the Nazi's, and we have progressed far enough now tech wise that it can be reverse engineered and/or put to use.
I think there is a huge amount of historical and archaeology evidence that is hidden away. The truth IS stranger than fiction.