More recent history seems to paint another picture - knowledge is quickly gained as cognitive ability increases, and is even more rapidly lost once it diminishes.
Think of not only the replication crisis in science, but also the despair to retain "tacit knowledge" the failure of which often results in outright abandonment and outsourcing to somewhere else.
The technology to land on the Moon seems to be effectively lost. The supersonic airliner is for some reason not available either.
Same happened in Rome, and Rome itself together with Greece rediscovered civilization from a much darker age before them.
In light of that, the idea that any real knowledge could be retained for that long is preposterous.
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