I have a suspicion (based on absolutely no evidence, other than that I'm an Engineer, and expect Apple has some brain cells); that the way this feature works is with a magnet and reed switch to physically disconnect the lines with no "communication" via software/firmware or other electronic means;

Based on their "hardware only" wording for the newer devices, I'm inclined to believe this is the case.

There exist easier targets, such as the loudspeakers which can be turned into microphones.

https://www.usenix.org/system/files/conference/woot17/woot17...