I need to agree. Germans take a lot of pride in Engineering as a culture. To say the German Engineers took short cuts just to pass US tests seems more unlikely than a strict Wink-Wink-Nudge-Nundge from the Bosses to the engineers with the side effect of or-else.
Completely agree. Not to mention that most engineers work to a functional specification. The software controlling what the emissions control computer reports is a pretty simple concept: pull readings from the on-board sensors and push them onto the output bus. Anything that deviates from that would need to have been driven explicitly by somebody. Code that detects emission testing equipment and conditions doesn't just get added by a couple of engineers on a whim.
I'm sure that a program manager was given the EPA requirements and told "You must meet these (by any means)." That PM passed them on to the engineering team with clear instructions that the limits must be met, one way or another.