Presumably for a car to be sold from factory and registered by owner as street legal in Germany, it must not be able to make loud noises under reasonable operation — specifically, driven like someone would drive a car (up to 100% throttle, up to redline rpm) but not attempting to maliciously create problems (fuzzing the pedal inputs for backfire scenarios from factory). If the police are able to
reasonably operate it on a dyno to create an illegal condition, then either it has a mechanical defect, an owner modification, or a design defect; and in all three cases, the vehicle would presumably therefore
not be street legal, as proven on the roadside dyno. The loophole you’re describing wouldn’t hold up in front of a judge anywhere at first glance: “I don’t ever push the gas pedal that far down in my fancy sports car designed/modded to accelerate loudly when I push the gas pedal that far down” is not a particularly convincing argument when one is found to be operating said vehicle. A more convincing argument would be, “the pedal inputs by the police were improper for operation of a vehicle and do not conform to the training given to roadside dyno operators”; and, presumably dyno logs and body cam of the operator are available to defeat that as well. And arguing that the training itself is improperly having police operate the vehicle through all factory-provided regimes would almost certainly fail (even in the U.S. were such roadside dynos a thing here!), but if your case was that the training needs to be improved in a specific way
rather than for termination of the roadside dyno program, you might get traction with that.
One assumes this process is generally reserved for vehicles whose driving has earned them a dyno inspection, as it would induce mockery were they to e.g. dyno an old beaten-up Geo Metro without cause, and that the fines are directed towards either the vehicle owner and/or manufacturer, as circumstances require, as the responsible parties for its mechanical-design and/or aftermarket-modded violations of the law. (I am not your lawyer, this is not legal advice.)