This is correct - @OP this is no worse than the accuracy using standard bike computers with GPS and speed sensors. When I bring that data into Strava, Strava compares to their built-up database from innumerable rides tracked with various devices, and apply a correction factor, usually no more than a few percent. How does my computer, or strava's averages, or your Apple Watch compare to ground truth? Impossible to say without actual survey equipment. Car odometers are only good to 1-2%.

In short, don't worry about this part, it's fine.