Not true IMO. The speed sensor is the same hardware as a wired bike computer, and you're probably thinking that it's only used for measuring instantaneous speed and the GPS is used for overall distance.

The wired bike computer properly calibrated is much more accurate (in the case that the GPS ignores the wheel sensor for distance) because of the numerous sources of error of the GPS. Primarily among them, the transient variance of measured position (5 meter accuracy) by the GPS means that it only approximates the actual path travelled.