It doesn't seem unreasonable to you that they don't know how the throttles work?
Not excusing anything, but don't forget that the handling pilot was fresh off flying an Airbus for some time, where the throttles don't move when the autothrottle is engaged - stupid in my opinion, the automatic "hunting" of the throttles on a Classic Boeing 747 ( no idea of the T7 ) was a visual clue (and tactile one 'cos we had our hands on the throttles anyway) that everything was doing what it should do - so 'thinking' that the autothrottle was engaged, as I believe he has stated (?) then he wouldn't be concerned that they were stationary. Old habits, new tricks etc.
Only one aspect of course, what was his thinking about the falling speed - if he was thinking about it at all ! - and what was the other pilot doing.
Life was easier before computers - in every walk of life !
We just flew the bl**dy aeroplane !