Anything can be in the firmware, there's no reason the payload signal has to be processed at the same point in the signal chain as everything else. And I'm not saying that it needs to be a complete fly-by-wire system, just sufficient to be able to ground the craft, which seems trivial to me - all you'd need to do is to just stop processing all signals on the bus.
I can 100% believe that the consortium of corrupt liars and incompetents that make purchasing decisions could absolutely miss this - either on accident or on purpose. Where do you derive your unshakable faith in leaders?