He means Markdown, but Markdown is mean to turn into HTML. Rendering it any other way than converting it to HTML gives a different result, and if anyone complains about it, it'll be your fault since you didn't render it the proper way. It turns out a lot of things are like this. Another commentator mentioned Slack messages.
I feel like Typescript is a good analogy here. Typescript is its own language, but it is intended to be transpiled into Javascript. Yes, you could probably create a JIT that executes Typescript directly—but you would do so at your own peril. Reaching for an existing Javascript interpreter such as JavaScriptCore or V8 is the logical choice.
