please recall that when you select flaps 25 in the beast Approach Idle is automatically selected. There are several other parameters for this selection but none the less this should give you a fairly rapid spool up in the event of a rejected approach or landing.
However, we do not yet know his configuration at 600'. It it possible they were at or near idle and selected Flaps25 just prior to that, i.e., after the "160 at 4" gate, and the throttles "failed to respond" after configuring and decelerating? Indeed, if the Pilots were at flight idle instead of approach idle, the lack of response could be a bit alarming...