For me personally, I think there is alot more to the history of the human occupation history of earth. I think much of our prehistory is buried under 60 feet of water and silt that used to be the coastal regions of the continents 10, 12, 15 thousand years ago. Then the ice dams of 3 mega lakes break and we get destruction of those areas, flood myths, and a reboot of history from a survivor perspective. BUT, I think that is a far cry from E.T. presence.
Especially since our knowledge of mining methods and ore extraction efficiency was ridiculously limited. If we were bred to mine, we should be bad ass minors. Physiologically, we should be short, furry, muscular people with excellent low light visual acuity. We should also see evidence of very efficient mines, ore processing centers and rapid transportation pipelines to deliver the semi-processed ore. Especially so if you had the help of advanced technology and techniques.
Instead, we range in height, are only suited for warm weather open air environments, have poor low light visual acuity. Not to mention that we have a historically demonstrable inefficient methods of mining, ore extraction, ore processing, ore refinement, and bulk ore transportation pipelines.
If Aliens bred us to do anything, the case could more easily be made that it was to be warriors or artisans not workers. Sitchen's stuff is interesting, but even the technology he talks about is state of the art 1960's stuff. Rockets, long landing strips, and nuclear bombs. The average tech savvy person can look at the technological assumptions that helped form Sitchen's conclusions from the 70's and 80's and immediately spot the problems. Knowing what advancements in technology have been made since then, his interpretations seem... how do I put this... old, outdated, and vastly insufficient to pull off the operations he assumes they are doing. This is the trapping of a technological forecaster. You can not peer into the crystal ball of plausible technology with any sort of accuracy anymore. Our rate of discover and techvolution(tm pending) is just too great.