We are both right 😆
It is true for Signal on Apple devices.
It is not true for Signal on Android devices*
*Well I'm using grapheneOS so I feel more comfortable in my case but a regular Android device with full access Google Play Services? That I'm not so sure about. It's conceivable that Google has a way to read the final notification (FCM push -> Signal fetches and displays message -> Google can read all notifications on the device, FCM or otherwise) 😬