Yeah, I initially wondered about a botched single engine failure. It's not uncommon in twin piston and turboprop aircraft for someone to shutdown the good engine. It is less common in turbofan aircraft because their performance is typically pretty good, even one engine out, and there is no immediate need to manage the dead engine.

As you say, the complete lack of any unintended yaw motion and lack of any evidence of failure of either engine in the videos make it seem less likely.

An auto-thottle cock up does seem like a likely, if scary, scenario. Two competent pilots on a CAVOK day losing a plane under complete control just by failing to advance the throttles manually. I'd prefer a mechanical fault.