Gradients are used a LOT in this app. This nav bar gradient effect is becoming pretty common throughout first-party apps as well — first with the iOS 17 Health app, then Journal, and now Sports. watchOS 10 is full of gradients. (Actually, the Journal app is _entirely_ gradients!)
Take a look at the “Yesterday / Today / Upcoming” picker right above the list. This is functionally a segmented control, but they did not opt to use the stock style. Instead, they’re making heavy use of the system-provided vibrancy effect.
The default control is blockier and stands out more, which wouldn’t make a ton of sense in this context where it isn’t a super important control.