The whole _point_ of implementing these APIs is to allow the user to reply using a different app; replies direct from the notification on the device itself are a hook to encourage developers who don't care about the rich integrations to allow that functionality anyway.
And that's one of the things I really like about Android: "platform" apps like WearOS don't use private APIs, they use public APIs so other developers can build on platform features. I have seven apps installed that ask for notification access, five I've granted.