This argument doesn't make sense, Signal is fundamentally not an anonymity tool. It's a privacy tool.

They intentionally store very little data about their users but their "mailbox" model explicitly means that the server (or an attacker) can figure out who is talking to whom. And that's fine! Signal isn't trying to be an anonymity network, so while stuff like sealed sender are nice features it's important to remember that anonymity breaks are explicitly not part of the Signal threat model.

"Don't stand out" only makes sense as advice for anonymity tools to avoid bifurcating the anonymity set -- it doesn't make much sense when you're taking about a service which links your account to your phone number and thus every server request is explicitly identifying you.