I think they mean at the end of the clip they drag the slider from HTML to Typescript and it puts all of the blog data into structured Typescript data then writes a function which will generate an HTML page from that. The resulting blog output will still eventually be HTML, it's just whether the entries are made as data segments automatically stitched together or entered via raw HTML formatting.

I used OpenAI Canvas to help with the conversion. Essentially, I took the static HTML/CSS and wrote TypeScript to dynamically render blog posts.