Signal does NOT send notification content througth APN/Firebase, their push notification is literaly a ping as you expected.

Source: https://mastodon.world@Mer__edith/111563866152334347 (Meredith Whittaker is the current CEO of Signal)

I can't link you rigth now to the actual code on their repo but it is verifiable.

Btw I clicked your mastodon link and it didnt work