Not necessarily germane, but Bombardier auto throttles, Honeywell or Collins, in FLCH, will either command idle thrust or climb thrust, depending on whether the FL in the selector is below or above the present altitude or flight level. FLCH can control speed ONLY with pitch, so those thrust selections make a certain amount if sense--idle if going down, climb if going up.
To the present case, if they were in FLC, the autothrottles might hav been commanded to idle and might never have changed to SPEED mode. I do watch the A/T FMA and make call-outs of its mode, just like I do for FD and AP modes.
Not Boeing pilot, so pure spculation.