If you had a choice of
a) 99% chance of evacuating with a couple of broken arms and legs (and no real fire), or:
b) 1% chance of 20-30 people burning to death because of a delayed evacuation
It's the combustion byproducts (CO, HCL...) that can incapacitate in seconds. Demise from inhaling toxic fumes generally precedes combustion.
The real question in considering evacuation is less the state of the fire 90 seconds from now than the survivability of the cabin atmosphere 90 seconds from now.