Ironically, it makes me log in with my "username or mobile number" even though they know exactly who I am from the unique ID I'm trying to opt out of having :)
Without logging in, someone could enumerate all of these IDs and opt out everyone. This at least protects some of their spying.
Anyone could send that header and claim to be anyone they want to be. They couldn't use that header to authenticate you even if they wanted to.