It has nothing to do with assumptions about how smart people in the past were, we have every reason to believe they were just as smart as us. In fact we have direct evidence from surviving mathematical texts, treatises on philosophy, etc that they could be highly original and insightful thinkers. Smarts is simply not at all the issue and frankly you describing diligent scholars who clearly have huge appreciation and admiration of the people they study bigots is highly offensive and uncalled for.

The real bigots are the conspiracy nuts who think ancient civilizations we know of couldn't have achieved the many things we evidence of, and think it must have been aliens or other advanced civilizations on implausible magic islands.

The question is simply one of technology. We can't simply assume that people at a certain time had technology we have no evidence for, no accounts of, and no evidence of similar or near precursor technologies. The Antikythera mechanism was a shock because there was no evidence any such thing could exist at that time. We also don't have evidence such things existed in Babylon a thousand years previously, or in Egypt 3,000 years before that. Equally we have no evidence they definitely didn't have them. So should we assume they had such things? We simply make the best estimates we can and adjust when we find wonderful surprises like this one.