Yeah, this is partially the truth. The fenders are made to direct air around the wheels. Does the air go under the car through the wheel or out from under, at what speeds, at what ride heights, at what wind angles and speeds? Maybe a closer statement is "We just don't know wheel interaction with the air without extensive testing"?
On cars with aerodynamics for speed (think Bonneville) the wheels do not matter, spoke, disc or slotted has no aerodynamic advantage. Streamliner bodies smoothly push air around the wheels.
Tesla has designs to mitigate wheel aerodynamic drag with fender design and wheel placement in the fender. I have looked ad nauseum for reliable tests and mostly get opinion and conjecture. At least with wheel weight you get easily measured performance numbers for acceleration.