The study is more concrete than what you describe. Specifically they looked at questions like
“Do you think that over the last few decades our society has become less honest and ethical in its behavior, more honest and ethical, or has there been no change in the extent to which people behave honestly and ethically?”
So specific virtues are mentioned in the questionaires they evaluated, which counters the argument of changing morals over time. So your argument only holds if the morals in the time frame of the study (post WW2 USA) had shifted so dramatically that things like honesty had received a radically different valuation (which I don't think has happened).