I think the author means that they wrote a markdown document, which then was transformed (by a CI/CD pipeline, for example) to the html you see when you inspect

I think the author means that they wrote a markdown document, which then was transformed (by a CI/CD pipeline, for example) to the html you see when you inspect the source. I like your "Serve markdown, transform through a client-side script" approach though, so upvoting nonetheless.