A stationary engine cylinder can be considered sealed can it not?
Happily, I know more about engines than physics! Yes, the cylinder can be considered a closed container - during the compression and power stokes only, as a valve would be open during the other two strokes. That said. gasoline exploding in a cylinder, is what is intended by design, though 20 some degrees before top dead center is the intended time for that.
But gasoline does not pool in the cylinder, it either burns there, or is pumped out (where it can pool in the muffler, and explode there - caution about those live mag checks!). The danger is the unburned fuel dripping down through the carburettor, into the airbox, which is never a closed container. If gasoline is a puddle, it will slowly evaporate off the top, or burn as a pool of fuel. A pool of gasoline in the airbox will not be drawn up into the inductions system, which is why, in the case of a carb fire, you must crank the starter (mixture idle cut off) until you're battery is flat (the cost of the starter motor is secondary at that point). You have to pull all of the fire through the induction system, which means keeping cranking until the fuel is exhausted in the airbox. Yes, by design, airboxes are required to drain, but from my experience, that's never perfect.
In the case of the (C150) carb fire I had last year, (which looked the same as the photo), and from my wife's report, there had been enough gasoline in the snow under the cowl, that it too was burning - there was a tiny bit of charring on the nose tire, which I replaced. In simple terms, pumping the throttle allows you to pump out a lot more gasoline than pumping the primer, I think it's designed that way on purpose....