What’s reason we are talking about Signal here.

That’s very easy to answer. Signal comes up here because that’s what actually happened in the case that brought the bug to light.

The FBI recovered message content from the Apple notification service that happened to come from Signal, and used it to secure a conviction against people who used Signal in planning their anti-ICE activities

Unlucky for Signal from a PR perspective but that’s just how what actually happened, so people weite about that rather than hypotheticals where other encrypted messaging apps would have suffered the same issue.