as though using Signal means the authorities can’t read your messages.

Nobody who understands the topic thinks this.

Signal exists to prevent the contents from being read off the wire and from having the capability of compelling Signal's parent company from turning over stored messages.

It doesn't exist to hand hold you so you don't get phished, or prevent you from running it on a vendor phone full of spyware. You're responsible for making sure your hardware is secure and that you're not socially engineered.