Why is Apple saying that double tap is “new”?

The Double Tap feature indeed is new and exclusive to Apple Watch 9 and Ultra 2 because of the S9 Chip.

Your Apple Watch 6 Double Tap is the Assistive Touch Hand Gestures which can be located in the Accessibility of all Watches past the Series 4.

Learn more ⇢ Use double tap to perform common actions on Apple Watch - Apple Support

-Zachyy ✌️

The Double Tap that is available on the Ultra 2 and the 9 is a gesture that can be used in many different ways, depending on how you have it configured. On the earlier watches, it's part of Assistive Touch and is used to navigate back.

Use AssistiveTouch on Apple Watch Ultra - Apple Support

Use double tap to perform common actions on Apple Watch Ultra - Apple Support

It's like the button just above the volume buttons on the iPhone. On older iPhones, it does one thing - mute the phone. But, on the iPhone 15 Pro/Pro Max, you can chose what you want it to do from a variety of options.