Pretty sure that was SwingVision, an app based on real-time video analysis of your swing, rather than the hardware on your watch. The watch is just a screen to relay feedback to the user.
> the Series 7 can measure users' tennis serve speed or the distance of a golf drive, and "Hey Siri" prompts can tell users how far they've biked.
https://arstechnica.com/gadgets/2021/09/the-apple-watch-gets...