I am generally aware and wary of the hackerish tendency to deride mightily any intellectual undertaking or material that doesn't triumph, confirm, or conform to their strictly rationalist view of the world and its every corner. I find it boorish and stifling. More to the point, I have some incredibly intelligent friends who have spent a good amount of time with Deconstruction (not identical with Pomo, of course) and…
"Can everybody have truly been absolute frauds?" Yes. There are many historical examples of fields which are simply complete nonsense, with some practitioners who are well-intentioned and wrong and others who are just blatant charlatans. Alchemy is one obvious example. More controversially, psychotherapy in our own era probably qualifies ( http://www.amazon.com/House-Cards-Robyn-Dawes/dp/0684830914 ).
Alchemy had a long history, though, and often resembled practical chemistry much closely than it did a quest for chimerical substances. The practical side of alchemy produced some worthwhile facts and techniques, and overlapped in many places with early chemistry. Some late practitioners of alchemy are also honored as important early scientists: Isaac Newton, Roger Bacon, and Tycho Brahe among them. The http://en.wikipedia.org/wiki/Timeline_of_chemistry shows quite a bit of overlap.
But alchemy had to be rejected in order for modern science to emerge. There were several reasons: first, it was illegal to practice alchemy in many areas. Partly because of this, alchemists were secretive, while science advances by sharing information. Finally, Francis Bacon's work on the scientific method came along late in the game. So the only logical way forward was to re-brand the sensible parts of alchemy as natural philosophy or natural science, and proceed from there.