per the article

Signal can't access user metadata. It's not just that Signal promises not to keep logs. They've literally engineered their service to cryptographically prevent themselves from having access to metadata, even if they wanted to. Signal doesn't know what groups you're in, or even what Signal groups exist on the platform. They don't know the names or membership of any Signal group. They can't even access your profile picture or name. All of this is stored on user devices and shared directly from user to user. On the other hand, if WhatsApp gets a data request, Meta will turn over details about everyone in your group, exactly who sends messages to who, and when, because WhatsApp collects all of this.