"There are all kinds of reasons why a car has to behave differently while on a dynamometer and there are all kinds of special code branches that are executed only during test. For the programmers it probably was just another special case among many."

I think the point is that someone deliberately did this and they had their hands in the code. Yes, there are variants of the tune-able parameters for various regions and tests. As part of design and validation these can be used interchangeable on the test beds. However, someone, somewhere wrote the emissions defeator.