I find it odd that a publication has a blog. Is this content lower quality than what they usually publish? More meta? From a content management point of view it seems silly to have a completely separate stream of written content on a written content site.

Mirroring would imply it is being served from somewhere other than Medium, right? Is that the case or is this just a custom domain (medium.economist.com) pointing to their Medium blog? I believe custom domains are a common practice for larger brands.