Obviously, they are trying to capitalize on the ignorance of runners. If a runner hears "certified", they assume it is an accurate course. The RD is likely hoping that the runners believes it is USATF-certified.
I believe one could take issue with the misuse of the term "Garmin-certified". I would bet that the Garmin company would not agree that the use of their unit constitutes Garmin certifying the accuracy of the measurement. Since their units indicate a degree of accuracy, which varies from over 100 feet to within 10 feet, I doubt Garmin would agree to the use of "Garmin-certified", if they knew about it. A more accurate term would be "Garmin-measured".