Oh, fuck off.
They heated cinnabar ore. You get mercury when you do that. These people had metal, mined, and could build vast structures that weigh more than any skyscraper did for millennia after them.
You don't need a supernatural explanation that they found a liquid metal (a liquid mirror, in effect) fucking intriguing and so prized it as some kind of treasure to bury with their kings.
That people in these ancient eras had brains seemed to be frowned upon, as if we're the only humans who could be allowed to do that. Ancient Greek, ancient Egyptian, etc. civilisations all had astounding knowledge and abilities. Just because they were never able to fully capitalise on them and then we suffered a few thousand years of poxy ignorance doesn't mean they weren't geniuses. (Just so happens that several of those millennia were dominated by religious shit, Crusades, etc.).
Antikythera (extremes of "clockwork", gearing and mathematical technology), pyramids, battery technology, steam-powered engines, railways, they had a shit-ton of expertise, but the problem was that the insights were few and far between and hard to do, and secondary to surviving for the most part, so unfortunately they never were able to be joined together in the way we could do now.
Fuck your aliens. Pay your respects to thousands of years of education, science, inquisitiveness, some of the greatest minds who ever lived, single individuals who knew all of established science for their time, amazing insights, and artisans capable of creating their off-the-wall ideas using some of the most difficult craftsmanships in existence.