Except, they don't work with actual source code. An engineering team responsible for adaptation for certain market, most likely works in calibration space, that's it, just data (flags, thresholds and tables, thousands of them). To access this data they rely on ASAP2 description databases which are simply very large text files [1] containing ROM addresses, parameter names, etc. I don't know the whole VW workflow, but can imagine how one could define a set of switching parameters for cheating, and then delete their definitions, leaving these parameters invisible.