1300 paying users for a markdown app, I'm actually impressed by the size of that market. That's either a great job marketing or a much bigger market than I realised.
Personally I have never found the need for a markdown editor (never really liked markdown much for typing) but I can see clearly why lots of people like it.
What makes this a "huge market" for a Markdown app if everyone uses the not Markdown based note taking app shipping with macOS? I would guess that dedicated Markdown based editors is mostly a niche market, even if it seems to be common in the tech bubble.
I would assume that the use for a Markdown based (developer focused, as it seems from their landing page) editor such as this one would have a more niche market. Without conducting market research into the topic myself, I would say that his success with this product would serve as a pretty optimistic point that there is a need and use for a product like this.
For me anyways, the reason to adopt it wasn't to do with markdown, it's just about having better UX than the built in Mac notes app. I don't know why, but I just felt like I was fighting with it on certain tasks, such as using it as a scratch space for system logs or taking notes when doing customer support.
The markdown support is nice / simple for formatting, but at least for me that wasn't the reason to adopt it.