I am aware of sealed sender, I even mentioned it in my comment. ;)

My point is that by construction they are not an anonymity tool, no centralised server can be. They can add features to make it less easy to track users but that's not the same as providing anonymity, and it's important to not confuse the two.

To give an example: my email provider has a feature to auto-encrypt all inbound emails to a PGP public key. Is this a nice feature to make it harder for someone to read my past emails if someone gets access to their infrastructure? Sure. Does this make my emails end-to-end encrypted? Absolutely not. In a similar vein, Signal making social graph correlation harder is a welcome feature but is not the same as making it anonymous.