Earlier quoted context omitted.
I think Apple lets third party watches show notifications but they're not allowed to take any action in response (eg having a snooze button when your alarm goes off or whatever). Next version of iOS will allow interaction for a single smart watch at a time in the EU. I think it's still unclear if Apple will enable that feature outside of where it's legally required.
Interestingly a Garmin can turn off an alarm on an iPhone. Not sure if this counts as an action in response to a notification, exactly.
Stop alarm is presumably one of those basic actions.
But it doesn’t support custom actions that the app developer might have registered.