I find it interesting that VW are throwing their software engineers under the bus over this.
This just isn't something that a couple of developers could independently do, unless VW's oversight and change control is absurdly sloppy. It's going to require at least someone involved in the testing telling the software engineers "we have a problem, can you do something to make the car pass in the test cell" - software engineers don't just generally add code like this for fun. One would imagine there would also be code reviews and audit given that engine management system software is now a safety of life issue (think drive-by-wire throttle, which this code would likely touch).
While it's probably true that Cxx level management weren't aware of what was going on, I find it extremely hard to believe that at least some layers of management were not actively involved with this, and people auditing the software were not actively involved in this. If they weren't, and really just a couple of software guys can put code into the engine management system with anyone being the wiser then this is probably worse (and then the Cxx level people definitely carry some of the culpability for allowing such a sloppy regime in safety critical systems).