Someone's getting lost here. Isn't it also a foul if the B-DB who has the A wing back who is just off of the shoulder of the tackle, realizes at the last minute that the "departing player" didn't depart and he leaves his inside man and matches up at the last minute with the "departee" so as you note "he's covered". Then just one count later the QB throws a quick out pass to that now uncovered wing for a TD? The fact is they still confused the defense, resulting in a defense that had already "matched up" with the offense being either wrong or mis-matched somehow, and this is still a foul by rule 100% of the time.
The single fact that the non-departing player is covered is no where near the whole story and is only one part of the judgment that we have to make here. We need to judge that the actions of A did not confuse the defense as a whole, and ultimately A got a net zero advantage as a team. In my opinion, if we have a defense that has been called, in position and waiting for the snap, that realigns at the last minute trying to figure out what they need to do because of a "false substitution", we've still got a flag.
Well, then, let me be your compass. I agree with you. But, in the play being discussed, no one suggested that there were any other mis-matches, although mis-matches in the interior of the offensive formation are going to be more difficult to define. If the tactic A use clearly confuses the defense and they aren't able to match up - within our ability to determine match ups - then A has fouled. But, in the play given, it was more like the DB initially appeared to have no specific assignment, i.e., "his guy" was missing; then, shortly before the snap he notices his guy and moves out to cover him, with no other effect on the defensive formation. No confusion, no foul. But, as you suggest, if the defense overloads to one side due to the offensive formation - at least what they can see of it - and they aren't able to fully adjust to match up, then you could justify a call, if the unmatched player has significant impact on the play. Easy on an uncovered wide receiver; a bit more difficult - but not impossible - on an inside receiver.