When you reject a takeoff and bring the aircraft to a halt the pilots are unable to see anything inboard of the wingtips without opening windows. They may or may not have received a left engine fire warning indication, it depends on whether the fire loop for that engine detected heat. If there was a rapidly expanding puddle of jet fuel on the tarmac that was burning and the flames were going on to the fuselage there would not be a fire indication.
I am sure the controllers were extremely busy coordinating fire services and go-arounds.However if the tower had a clear view of the aircraft would it not have been useful if the controller had told the aircraft that there was smoke and flames billowing from the left engine? Armed with that information the evacuation might have been initiated earlier.
I know that if I were to reject a takeoff, having informed ATC I was stopping I would be very grateful for any immediate feedback from ATC as to whether they could see any smoke, flames, inferno etc. Fortunately in the absence of any ATC info there was an extra flight crew member on hand to go back and see what the passengers and cabin crew could see.
If a passenger had panicked and opened door 2 left off their own bat there might have been a very different outcome. As it was what a great outcome and hats off to all concerned.