They were being polite because you were flat out wrong: a walk through a shopping centre gives you absolutely zero information about whether prices chosen have been tested or were picked by throwing darts at a calculator. It just tells you what prices those shops have chosen to sell things for, nothing at all about the reasoning behind those prices.
And the OK response is similarly being nice because if one thinks that the retailers for the past 100 years haven't extensively tested pricing, both in the real world and via simulations, they are naive to a large degree as to how businesses work.
I think their comment is pointless, and would agree with you calling it a non-sequitur, however I feel the need to pedantically point out that when you wrote "... it seems really bad logic to assume that...", that is explaining a human action (assuming).
/s