How many editors can you think of that don't do any (strictly) unnecessary file operations, e.g. renaming files? The author's problem is that Emacs is just really good at it, so he wants to use it more.
It gives me fewer reasons to leave Emacs. Which increases my productivity. There's very little 'mental context switch' delay since it's all the same tool with the same interface.
I'd call it more than a shell. It's more like a text oriented Lisp (Emacs Lisp to be specific) runtime environment with built in editing, interpretation, compilation, and debugging tools. It's also been a popular home for numerous applications that have found lives of their own, far beyond editing text.
I like to call it a generalized interface to information.
The web of the early 1990s could be called the same thing, but not the modern web, which is more like a "generalized experience-delivery platform with an old much-simpler generalized interface to information at its core".
An OS may be too much but, as a proof of concept, there are people who have been running Emacs as PID 1. And I don't mean in an OCI container (we're used to have only one process there): I mean on the bare metal, with Emacs being PID 1. A bit radical but it's been done.