That's exactly why I'm so baffled that some people have a problem with it.
It literally happens in one scene for a second, if that. If I blinked or talked to the person next to me, I would have missed it.
Even more baffling is that they show that these people EXPLODE in certain conditions and that heat and fire is largely involved with extremis . . . yet, to some, a fire breathing individual is just too damn much for someone that can control the high levels of heat with their condition.
So they can spontaneously combust if they get too hot, heat metal with their bare hands but a second of fire out of their mouth is going too far? I just don't get it. It's like saying that you can believe the T-1000 can turn it's hands into knives and stabbing weapons but mimicking another person's appearance is the straw that broke the camel's back.