They had nothing to compare the clocks against, and I don’t think the stars and the sun would have made an useful target for comparison.
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The other thing that I haven’t seen mentioned (at least not explicitly) is that the development of modern, accurate every day clocks was a gradual process. It wasn’t a situation where someone said “here’s this thing I just invented called a clock that will perpetually keep the correct time.”
Prior to accurate time keeping methods, time was an estimate. Simple time keeping tools like sundials can get you in the rough ballpark. Early mechanical clocks had to be frequently reset to stay on time (I think typically adjusted at noon based on the sun). Over time, they developed new mechanisms until they got more accurate.
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