True, the AFM doesn't produce voltage. The question is really, what signal,
measured in millivolts, does it return to the ECU?

This is important because the signal returned to the ECU, along with other
data, tells the ECU what air/fuel mixture to send into the cylinders.

It seems to me that for any given set of conditions (say, trying to
accelerate  up a 3% grade at 3100 RPM in fourth gear) the signal returned to
the ECU should be constant.

Otherwise, the engine will be running too rich or too lean.

If the millivolt signal didn't really matter, it seems the VW engineers
could have designed a device much less complex than the AFM, or eliminated
it altogether.