Does that mean receiving addresses are known to Google, by default?

Does that mean receiving addresses are known to Google, by default?

Yes.
The core reason chat apps need push notifications is that mobile OSes don’t allow apps to stay active in the background. Without push, the app can’t receive messages or notify users when it’s not open.
Notification services have become increasingly centralized, and we’re deeply dissatisfied with that. It’s up to each user to choose whether to enable or disable message notifications.