As much as I agree that it’s possible to work on the Apple Watch Ultra one, the battery hit would be too large. From what I understand, the Apple Watch Ultra series one would need to use the CPU to listen for the double tap. On the Apple Watch series 2 it uses the Neural Engine to listen for the double tap, which is significantly less power.
That is why Apple is not releasing it on the older watches. The neural engine just did not exist a year ago that is capable to listen to this option when invoked. The Neural Engine just sits there in the background and quietly listens/monitors the entire time the watch is on for someone to double tap. If you tried to do this with the CPU on the Apple Watch Ultra series one the battery would drain way too fast.
When I turned on the accessibility feature on the Apple Watch ultra series 1 to mimic double tap, my battery drain was significant as the watch was constantly listening for a gesture.