What a malicious sensor could do is store user's fingerprint for retrieval by unauthorized parties.
Of course, taking advantage of the exploit in question requires the phone to be stolen by an extremely sophisticated (if not state-level) bad guy, altered by installation of a malicious sensor that has never been documented to exist in the wild, then recovered by the owner, and then stolen again at a later date. All to acquire personal biometric data that could just as easily be obtained with a piece of Scotch tape.
A simple application of Occam's Razor suggests that Error 53 isn't a "security feature" at all, it's just Apple being a rent-seeking asshole.