Agree. A lack of instrument scan, simple as that. Pilot 101.
This crew obviously did not monitor their airspeed. Had they noticed the decaying speed, all they had to do in a Boeing - in any aircraft for that matter - was manually push the thrust levers forwards.
If the speed starts dropping, I glance at the engine N1 or EPRs, and I expect to see the auto-thrust bring the engines up to correct it. If auto-thrust doesn't react I do something about it.
If the automatics are not doing what is required, you step in and reselect or take over.
Unless, maybe the thrust levers both became physically jammed at idle and both crew struggled to release them as the speed decayed, but we haven't heard anything along these lines.