I'm quite surprised by the negativity here. If I was commissioned to make a custom blog site for someone, I would probably go down this route, and use a headless CMS to make it easier for others to generate content. But using plain markdown is not a bad choice if you can't be bothered with, or can't afford a server.
With the stack described as I understand it, the front-end flexibility is preferable to what the likes of what WordPress will give you, and the stack will allow a developer to decouple the CMS backend from whatever plans he or she has for the frontend.
I suppose one can complain about React all they like. I'm not the biggest fan of React, but component based rendering has it's advantages, so I can't see the issue with that choice.
The only thing I don't like, is 'JS all the way down'...
Not because I hate the language (I think the language is fine, and TypeScript is overrated). But just because I'm still not quite sure what the effect on SEO and accessibility might be, when developers go 'full JS'.