Surely if you had a fire that big and were for whatever reason mithering over calling an evacuation you'd run the evac checklist down to shutting off the engines so you're as ready as you can be - and if required can instantly make the "EVACUATE" call? Why would anyone not do that - unless the rabbit in the headlights scenario that I suggested a long way back was correct.
It strikes me as very odd indeed that either engine was still running - they surely weren't starting the APU...?
They don't seem to have even been ready for an evacuation, and that's very odd indeed.