Yes, Signal does not pass any message content through Apple servers. What's happening here is that notifications are stored locally on the phone, and if those notifications are set to display name and content they will be retrievable from local storage on the phone. Digging into this now but it seems the notifications are stored locally for 30 days (no idea about guarantees of deletion after that).
And of course even if you yourself have previews turned off in your notifications, the people you are sending messages to may not.
Meredith Whittaker addressed this in December 2023, saying “push notifications simply act as a ping that tells the app to wake up. They don't reveal who sent the message or who is calling (not to Apple, Google, or anyone). Notifications are processed entirely on your device. This is different from many other apps.”
https://mastodon.world/@Mer__edith/111563865413484025
