This is all kind of amusing. As a passenger, if the aircraft is stopped on the ground and I have any reason to suspect there may be a fire on board, I'll initiate my own evacuation. The cabin and flight crew can stay on board and argue about protocol if they want, but I'll be outside the aircraft and safe in the meantime. I don't care about regulations or legal consequences in emergencies. There's a reason why all passengers have access to instructions that explain how to open the doors.
I do know one thing: a fire can be blazing away in an aircraft even before the smell is noticed in the cabin (much less on the flight deck). And a smell can turn into a cabin filled with flame in a few seconds. So I absolutely cannot blame a FA for initiating evacuation—better safe than sorry. And if she hadn't, I would have (at least for my own evacuation, others can figure things out for themselves). Her only mistake was not making clear to the flight deck that an evacuation might be a good idea. After all, they are in danger, too, and if they discover a fire too late, they might not make it outside. Aircraft fires do not follow chain-of-command protocols.