I'm not sure I understand what exactly happened here. Was it previously possible for non-apple engineers to replace the home button or was it not? The guardian's article seems to suggest it was: "Indeed, the phone may have been working perfectly for weeks or months since a repair or being damaged."

If that is the case and it was possible to replace these sensors before, apple's narrative that the "error 53" code was introduced for security reasons doesn't seem to make a lot of sense: If the hardware sensor wasn't designed with secure authorization (e.g. via asymmetric cryptography) in the first place, all they could do now in a software update would be to add some kind of cosmetic device ID check.

However, any such newly introduced check in software could not actually prevent "malicious sensor" attacks but would only add a (possibly trivial) additional step to the attack where you have to spoof the correct device id.

Or maybe my reading of the guardian article is imprecise and replacing the home button has always meant loosing access to at least some security-relevant features?