The general theory of relativity predicts that time would run slower, and if that was the case these ancient quasars would demonstrate certain properties when l

The general theory of relativity predicts that time would run slower, and if that was the case these ancient quasars would demonstrate certain properties when looked at by our telescopes. We then looked at ancient quasars, and the readings we got matched the predictions, with some amount of error. This can then be used as another piece of experimental data supporting the general theory of relativity, even though you are right that these readings could be achieved through some other bizarre set of circumstances.