Well, not quite. Since the incident measurement can't know anything about the grey card used in the reflective measurement, it needs an absolute intrinsic reference value. If both measurements agree, it means that the reference for reflective and incident metering are the same. If the reflectance of the grey card is 18% then this is the common reference - at least for this meter. By the way, this is only one meter allowing for both types of measurements.
Now, whether the 18% standard is appropriate to putting it on zone V is another question. This is much more subjective in practice and has little to do with the densitometry. At least for me it works fine.