Cabin crew should've initiated the evacuation, even if the fact that the fire was not yet known to the pilot..
What rubbish. As others have already said, the pilots may not have shut down the engine(s) straight away if they weren't planning an emergency evacuation. I'd suggest that would be the case if "the fire was not yet known to the pilot". An evacuation initiated by the cabin crew in such circumstances could easily have resulted in people going down the slides right next to an engine that was still running. Is that a good idea? I think not.