OP: Haha, no, it's not fake.

It's done via a Wikipedia feature called "User script", which works like a Chrome/Firefox Extension. If an user choose to install what you build, it could load additional JS and CSS. Our code is open sourced on https://github.com/wikiloop/signed-stmt, and the feature is enabled for my own account User:Xinbenlv https://www.wikidata.org/wiki/User:Xinbenlv/SignWithEtheruem...

Anyone interested could try to enable it in their account if they choose to, by opt-in this user script that we built.