Would that mean that, hypothetically, a bad actor could convince the manufacturer to place sensitive data about you in inaccessible parts of the disk? So short of destroying it completely, a computer (especially one where permanent storage is soldered in) could be assumed to contain anything that was stored on it even once?

Well when the government will become very interested in hoarding old electronics and registering every facility able to destroy them, we should become suspicious.

I would be very relieved to be proven paranoid.