That it was not the autopilot misbehaving but the MCAS. The pilot probably assumed it was the autopilot (and therefore deactivated it) but didn't know about MCAS in this situation.

I wonder if the system showed a warning that was active to give the pilots a hint of why the aircraft behaved the way it did. Even if they didn't know about MCAS, a warning that it is active would've at least triggered further responses (e.g. cut off power to the systems to return to manual control).