Could it have been a pilot in a nearby aircraft, saw there was no fire, spoke up but then decided he didn't want to stop the evacuation since it seemed the safest thing to do and that Captain K has already began it?

There was a limited number of people using the radio on that freq at that time that could see the aircraft and respond in that way.

Or perhaps one of the Fire/Rescue folks that saw there was no fire, then backed down for some reason?? It probably wasn't a tower person, they were already part of the conversation with the pilot and they too wanted to know who the unknown person was.

Interesting that the unknown voice didn't speak anymore after the request to identify himself. Makes me think it was someone that decided they didn't want to interject themselves into the situation.

Looking forward to hearing how Allegiant management spins this. They probably will say he did it as a pro-union thing, something to encourage management to agree to contract, etc.