Is it still possible for a manufacturer to refuse to honor a warranty if that is the case, namely that the device was damaged by a botched repair, and this just says that they can't make that determination based on a sticker alone?
An analogy with cars, where the same law already is being applied, will work. (Do we still do car analogies in 2018? And on sites which aren't Slashdot?) Suppose you get your brakes changed at some place which is well-known for brake repair. Then, later, your engine dies but the manufacturer's warranty should still be valid. Then, if the manufacturer wants to claim that the third-party brake repair is the cause of the engine failure (and thus that the warranty is invalidated), the burden of evidence is on them.
They will need to prove the failure was more likely than not caused directly by aftermarket part, I am actually surprised they are taking it that far instead of of just offer some kind of fractional settlement, i.e covering 80% of the repair or something.