I have an e-bike. Just to test the GPS accuracy of my new watch i used the "bike" sport program. The top speed of my bike is around 20-24 km/h. The GPS tracking went really well, I am impressed there. However I saw rather strange and high heart rate readings. It was 113 bpm in average and 146 bpm as highest. I actually do not have much physical activity using the e-bike in full assist mode. I really have to push hard myself to get my heart rate to about 150 pbm.
I did repeat the test and got the same result.
So I have to conclude that, the watch checks my speed and thinks I am pushing hard on a "real" bike. It does not take the actual sensor reading, but something "pre-programmed" = fake one.