On the online users list my account became 'invisible', that's true. But in real, if visiting any topics, if visiting my profile, if opening a hovercard of my profile, - everywhere my account is still 'visible' to anyone, since in fact the anonymity wasn't enabled by clicking on that button (icon), because that button (icon) didn't change my "Online Status" to "Hidden". I see it clearly in my account settings here: /forum/settings/account-security/
In case I change my "Online Status" from "visible" to "Hidden" manually, the icon will not let me change the icon to black, after clicking on the icon to make myself visible, and then refreshing the page, the icon will still be gray, and my profile continue being anonymous π