That assumption is probably not correct . A vehicle on a different dealer lot might just be there because it no longer meets the needs , that dealer has something that the person wants or it might be one of those people who just feel better having a vehicle under the warranty period.
I have never thought it was always true or even mostly true, I just think it is true enough to tilt the odds of getting a good car.in my favor. I know the two used cars I bought at a Chrysler dealer were in great condition and gave me many years of service with no surprises.