Pretty simple if you ask me.
The rocket engine is physically attached to the rest of the vehicle by some pretty stout mount structure, which transmits the F (= Ma) to the fuel & oxidizer tank(s), payload etc.
So yes, in the big picture, there is a summation of (pressure x area) over the rocket engine combustion chamber and expansion nozzle, and this force summation is the de facto thrust applied to the vehicle.
You might ask the prof about the shuttle external solid-fuel rockets - if there is no "force" involved, then what good are the solids?
