I certainly trust you. VW/Audi has stated this very thing many times what the repercussions are on modifying power-train systems.
That said, VW/Audi have also said, before an emissions fix is made, if an owner modification is discovered, the emissions fix will not be made unless the owner corrects the modification and provides an unmodified vehicle in for the emissions fix. So the PO having a tune done before the emissions modification doesn't eliminate having the emissions modification completed, provided the PO reverses the tune and brings in an unmodified vehicle. The issue, I think, is that the PO did a tune "after" the emissions modification was performed, and this subsequent modification was later discovered after the emissions modification was made, which carries with it the risk of voiding the extended emissions warranty.
When was the tune done after the emissions modification discovered by VW/Audi? After you took ownership of it? Or was it discovered while still in possession of the PO? If the latter, and if you discussed with the PO if the car carried an extended emissions warranty (which the PO would have known was already voided), you purchased a vehicle from an unethical PO, unfortunately. Not sure there is much you can do on a private sale like that though.