This appears to be a MJML wrapper with a Markdown→HTML converter attached to it. I think generating HTML from code is easier than generating Markdown, since there are many templating tools that understand HTML escaping. And writing HTML is not that hard, especially for your typical emails, so I'm not really sure if this library would be helpful in the long run.

I like the idea of this tool, as writing Markdown for some people is probably easier than HTML. I mean, use whatever floats your boat. I like that this exists.