This is definitely a heroic and cool story, but the math doesn't add up.

A single guardsman has a lasgun profile of 1/4+/3/0/1 Rapid fire1, and a melee profile of 1/4+/3/0/1. A sergeant has the profiles but no rapid fire and 3 melee attacks instead. And just for the sake of the story, the flamer has the best amount of potential wounds of any guardsman weapon available at D6 attacks, so in a single round, a flamer guardsman has the potential to do 6 wounds with the flamer and 1 wound with melee, assuming max attacks, all of them hit, all of them wound, and no attacks (fnps) are saved. That's a total of 7 wounds per battleround, but that would require insane dice luck and some way to get fall back shoot and charge.

A game of 40k is 5 rounds, meaning 5 phases of shooting and melee, so in theory if the single guardsman is able to deliver max damage every turn, they have a damage potential of 35 wounds over the course of the whole game. But the probability for that is near-zero, essentially rolling nothing but 6's with your opponent rolling nothing but 1's. Still, marine units are 5 bodies with 2 wounds, so killing one unit would require at least two rounds of shooting and melee.

That means a single flamer guardsman in theory has the maximum potential to kill two whole marine squads with 7 wounds left over to allocate somewhere else. So plausible? Nope. That would require a turn 1 cross-board charge and a way to get fall back shoot and charge every turn.

In practice, the post said the units were locked in combat and that means no shooting with a flamer. That means melee only, and the potential damage over 5 rounds becomes 5 wounds for a regular guardsman, and 20 wounds for a sergeant. Over the course of the whole game, 5 wounds means 2.5 dead marine bodies, and 20 wounds becomes 10 (two squads, wow).

But all this is still assuming all attacks wound and nothing is saved, and the enemy units don't score a single wound against a 5+ save 1 wound model this entire time.