Flame is a specific damage type in CoH along with ballistics (tank shells) explosives (grenades/mortars/arty) and small arms.
Infantry entities used to have a small chance of instantly dying at even 100% health when they took any kind of flame damage. This is no longer the case.
I think because flame damage dealt small amounts of damage very rapidly (to simulate burning) it would create plenty of dicerolls that could at any moment result in instant death,. By requiring infantry to be already damaged to score those crits, the chances for sudden death is much harder to accomplish from flame damage alone.
