Lee, It's not an assumption.
When you close the MPD and see the mano reading drop (pressure increase), that's proof that the flue gasses are "piling up against the MPD". They get compressed as they slow down to squeeze through a smaller opening (restriction).
The same thing happens with the choke on a carburetor, but to accomplish something else. There it's piling up air outside the carb to cause more pressure drop on the down stream side of the choke butterfly valve (a carb's MPD). The extra pressure drop is to pull extra fuel into the slow moving intake stream of a slow spinning motor (starter motor driven speed only), thus richening the air/fuel mixture for cold starting a motor.