First off, air passes above and below, as well as each side, so a wing is affective above the car, and a diffuser below, so one will work without the other somewhat, but neither replaces each other.
A diffuser in order to work as designed needs to do several things. One to slow down the speed of the air, and two speed it back up again.
You can’t use a NACA duct to vent the air into the body work, that would be like a parachute under the car. It’s designed, or should be designed to work like an airfoil, faster moving on top, slower moving air under equals downforce. If you drilled a bunch of holes in the wing, besides the turbulence, you wouldn’t have a good separation between the faster and slower moving air.
Not having a wing means there’s lots of turbulence behind the car messing up how the air comes out of the diffuser, making less effective and giving it more drag….