This is commonly believed and seems to massively let VW off the hook. Almost everyone was overfitting their optimization to the test and thus not delivering real world lower emissions. That's not illegal, just unethical, particularly since they also lobbied to not make the test more realistic. But VW actually detected it was in the test loop and changed the settings. That's a defeat device and completely illegal. There were a few much smaller cases of similar things found in other manufacturers but nothing even close to the scale VW did it.