Nice to see Signal (not the default Messages app) in the Smart Reply demo.
I use Signal because I don't want Google reading my messages.
(The app of course also has a choice of encrypting the payload - it's free form after all.)
push notifications services can merely send instructions, and the visual notification can be an object formed completely client side.
They show up on all apps that provide messaging.
I don't want Google reading my Signal messages on their devices or mine.