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.
It's been always kinda weird to me that BT spans four layers, from antennas to volume controls. You'd think all the vertical integration should make it reliable and interoperable, yet in practice it's the exact opposite.
