Your argument has become circular, because the what Signal uses push notifications for isn't security-relevant: the messages are empty and used only as a wakeup.
They still contain information, though. They say when you're talking on Signal. Matched with someone else's messages at about the right frequency to indicate a conversation, they give a pretty decent idea of who you're talking to.
Signal still has to be linked with a proprietary binary, which contains lots of tracking code.