Using Google to deliver notifications to Android phones achieves Do Not Stand Out, an important property that it would be an "epic fail" not to offer.
If you insist, you can install a version of Signal that doesn't use this service, whereupon you will stand out, or more specifically your notifications will stand out from everything else.
If you insist, you can install a version of Signal that doesn't use this service, whereupon you will stand out, or more specifically your notifications will stand out from everything else.