I think we might be forgetting something here...
Opening the cargo doors is a no no before pax are off if fire suspected incase it makes it worse with the added oxygen. Fire crews are aware if this and so should pilots. Therefore, if there is doubt as to whether its on fire inside, the safest option is to evacuate before it gets worse. 300 minor injuries is still better than any deaths. If you can confirm that its not on fire then better to allow fire crews to inspect and safely disemark pax normally. Its already been stated here that its difficult to be sure but if that red smoke light is still on and theres doubt, its safer to evacuate rather than risk the fire worsening whilst pax still on board. Imagine the panic and injuries then, and also what we know it alls would be saying about why the hell didnt they evacuate earlier...
A320 TT 4500