> XMPP is an open standard -> Doesn't this apply to Signal, too?

The Signal protocol is neither an open (you cannot propose changes or extensions in an open process) nor has it been submitted to an standards body.

> Some developers claim not to track users.

With Signal you have to trust a single entity not to track you. With XMPP you have the freedom to choose that entity (including choosing yourself).

> if we assume people can choose a non-identifiable username, then we can also assume people can choose a non-identifiable phone number

This is just dishonest. It's a lot harder to obtain a non-identifiable phone number than to think of a random username.