The engines weren't windmilling. They were producing power. It is very normal for a jet aircraft to need an additional power adjustment as it approaches the runway. For some unknown reason the extra power called for was not able to be delivered.
Well, I suppose my question is slightly outdated, now that the working theory seems to indicate lack of throttle response from two otherwise running engines, I'm quite aware that under normal operating conditions, the engines would've been spooled up on finals. I should've phrased my query in more general terms, rather than asking about "the" pair of engines in question. I'm still curious to know if they would've been able to provide enough power to fly the aircraft (without the hydro/mechanical backups) in the event that they were windmilling.