As long as they use their own domain and have that registered with some other registrar they can trivially move the blog to any other hoster, including a random VPS. So while the current setup depends on github and cloudflare they don't hold much power over him.

It's not really fully-owned, but it's owned in the ways that matter most

I think the point is that it's not someone else's blogging service. If CF or GH die, you can port this to some other platform or your own server without losing anything, compared to e.g. blogger.