Evidence is: Signal still requires a phone# that is your unique identifier. Thus when connecting two parties, it is bound to have identifying metadata about them. (and that Signal still operates within AWS cloud, and is bound by US law: FISA, Patriot Act, etc.) How much more than this do you need?
It is just enough that this metadata be handled within the computing environment of Amazon. Their refusal for anyone use their own server and federate with “their” (as in captive) users also prevents anyone for using it in any other way…
If you dont see that Signal requires that its users use a strong-selector phone# in order to use the service, there is nothing i can do for you.