Not sure how they can ever measure calories accurately
The gold standard involves a breathing mask to capture your CO2 metabolization, which would be an inconvenient way to wear a watch.
Unless there's a proxy that can be measured via trace gases coming through the skin, all you'll ever be able to do is estimate based on heart rate. Perhaps the accuracy of that might be improved if you had it calibrated via a proper measurement, because a HR of 100 indicates a different amount of calories burned for a fit person vs a fat person, but if it was calibrated that when you have a HR of 70 you are burning X, a HR of 120 is burning Y and a HR of 170 is burning Z, your now-calibrated watch should be a lot more accurate (if still not perfect)
Biting the hand that feeds IT
