Totally true. A diffuser must be fed undisturbed/unturbulent air, and only then the lamellae can create an underpressure as the size of the cavety grows.
With the e85/86 z4, it has a huge windtrap on the underside in the shape of the gearbox area which is not covered up which'll create a lot of turbulence.
On the e46 and e89 bmw did cover up this area with a plastic undertray. But even with these cars the exhaust isnt covered up, so its not 100% flat.
The only thing a diffuser can do for the z4 (like the aero kit diffuser) is filling up the holes between bumper and chassis, reducing another stock windtrap