We know this was not the case because it was a different experiment. Different experiments that measure different answers. All experiments that have measured the speed of light in vacuum (directly or indirectly) get the same speed, c.
Like many here, you are blessed with ignorance and I don’t think I can drive that of you but I will try.
If some mass weighs 6kg on earth and only 1kg on moon, we wouldn’t say that the mass is ‘shrunken’ and hence mass is a relative thing. We know that weighing machines get affected by gravity. It is nothing to do with accuracy of the machine.
Likewise, if a clock measures 60 seconds on earth, it may measure more seconds on moon for the same duration. It doesn’t mean that the time has actually dilated but it is due the fact the clock gets affected by gravity. One may introduce something like ‘timeness’ (analogous to ‘weight’) to account for the measured difference in time (analogous to mass).
When electromagnetic radiation can get affected by gravity, why not the atomic clocks?
Of course, it is not easy to convince people to get rid of the ‘deep rooted’ scientific beliefs (like the case with religious beliefs!)
To quote again Stephen Hawking, whom I consider as my virtual teacher in cosmology,
“Any physical theory is provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory.” (A Brief History of Time)
My thought experiment, whose predictions go against the notion of constant speed of light, is open to interpretation by anybody and needs only minimum sense of logic.
It shouldn’t make Einstein any less. I don’t dare to discredit this great personality.
Though Einstein’s theory of general relativity proved Newton wrong, Newton’s theory still holds true in the vast majority of situations. Einstein’s theory helped us understand the universe better and helped us move forward in Science. But this wouldn’t have been possible without the Newton’s theory.
If we insist that Einstein’s theory is the ultimate and nothing beyond to think, out of our love and attachment to this great personality, then science becomes a religion.
(After all this, don’t come and ask like a child “Did you mention all this on your original post?--- Yes or No?”)