JD, we are not at cross purposes: The checking of the transponder's integrity (Mode C) is going to be done by a call of the pilots altimeter reference the radar display, right? Hence the RDP conversion of the raw (1013) info. The pilot has no other way of checking what his transponder is outputting (it is invisible to him).
Yakity's point refers to this invisibility of what the transponder is outputting. The means of checking is invalidated if the pilot has the wrong setting dialled up (for the piece of airspace he is in). As well as the other problem he refers to.