Which way was the wind blowing and how fast ?
If the captain positioned the aircraft correctly to put the fire downwind of the fuselage it was probably a good idea to hold the pax onboard and let the fire service professionals assess the risks. Having hundreds of people wandering around randomly at a firefighting scene is a very bad idea.
If and when the fire service considered the passengers to be at risk they could take appropriate action to protect and supervise an evacuation.
Let the professionals do thir job, it is what they are trained for.