I don't understand his point. HTML/CSS are fine if you know them but a lot of people have better things to do than learn those, so they use wordpress/medium/whatever instead of mocking about in html/css. Also, his blog is only text and has no functionality, so yeah, sure use html/css, but most apps like youtube/twitter have a lot of functionalities and they need js.
"And because my sources are in git, pushed to GitHub,GitHub Pages is my host."-right, and if github is hacked so are you. So much for not worrying about security.