I'm not a qualified pilot but if I sensed an evacuation was necessary and some unidentifiable person told me to hold off or not evacuate, I'd be backing my original judgement that evacuation is necessary if that person did not identify themselves within seconds.
Even if they did identify themselves and give a reasonable explanation, if I still thought evacuation was necessary I'd still back my judgement to do so (fire, smoke or fumes not visible to those outside the aircraft).
At the end of the day the Captain holds responsibility for the souls on board and the aircraft and the decision rests with them to make the best call they see warranted.
Injuries in an evacuation are unfortunate but a better result than a mass cremation.
Airlines need to back their technical crew to back their pilots in emergency, high pressure situations.
One day we will have cockpit and cabin CCTV fed back to the black boxes and a Captain not proactive could well see themselves to blame for lack of evacuation due to management and bean counter reprisal whilst not being around (along with pax) to even defend themselves in an investigation.
It's the Captain's responsibility to ensure the safety of their flight. It's management's responsibility to ensure the safety culture of an airline is first and foremost.