GitHub Pages used to be served under github.com subdomains (they were switched after GitHub realized this was a same-origin security hole), and said username.gi

GitHub Pages used to be served under github.com subdomains (they were switched after GitHub realized this was a same-origin security hole), and said username.github.com subdomains still redirect to github.io for backward compatibility. It's not unheard of for GitHub to rename inactive accounts, though: most likely, they gave this its own domain for something like SEO purposes (as it's content marketing).