Stacking Damage.
350k is the total damage you dealt over that period of time, not a single damage tick.
So, no, if you sit there over 10seconds and you last flamethrower tick is 10k (for example), over those 10 seconds you’ve done a TOTAL of 10k damage, or 1k damage/second.
With grenades or bombs your damage might be 1500 per tick, but if you can tick 10 times per second your total damage will be 15k (for example) or 1.5k per second. FT is bad damage.
^^ What nevets said, though to add, take a look at your combat log, It should open your eyes to what is really happening.