Crankcase pressure

A motor is just an air pump. It moves air thru the inlet and the outlet. Throw some fuel at it and an ignition source and it will spin on its own. A pump does not create pressure or vacuum. It moves a certain amount of air or fluid depending on how big it is, if you restrict the out flow the restriction creates pressure in front of the restriction. If you restrict the intake flow it will create a vacuum behind the restriction. When the piston travels down in the cylinder with the valve open it creates a low pressure area. Being the atmospheric pressure is about 14psi at sea level it travels from the higher pressure to the lower pressure. This is why engines at sea level produce more power than engines at higher elevations. Higher altitudes have lower atmospheric pressure to move for the high pressure to the lower pressure.

So yes, you have right idea that it is because of the pressure differential that the air moves thru the engine but the low pressure lies in the cylinder not the intake that is why there is no vacuum at WOT.