There are some other characteristics as well. With pen and paper, you can only move forward. That is, your only real option is to complete the draft. You can revise or rewrite when you're typing it in. Neil Gaiman writes this way; fountain pen and notebook for first draft. Cutting material is much less painful. You simply don't type it in.
The temptation to go back and tweak what you've written is fairly strong with modern tools. An author can waste hours, an entire writing session, on rereading what they wrote and making adjustments. That's an activity best left to after you have a completed draft in hand.
Right. You identified the prime characteristic for the CUI crowd: no context switching. Though Wordstar purportedly was superior for text manipulation.
Robert J. Sawyer wrote an article [1] on why he still uses it. Made me want to learn it, but I realize that if I'm going to put effort into learning a tool, I'd likely want it to be Emacs instead. (I currently use Scrivener.)
Emacs is worth it from a “creating an ideal environment” perspective, but it is a nightmare from a “stay focused” one. Tweaking your editor becomes very easy and very distracting.
I use emacs and org-mode all day long, and never tweak it unless there's a really strong reason. My goal is to keep it as close to vanilla as possible.
I use emacs as well, and regularly get distracted tweaking it. Maybe the temptation to tweak isn't present for you - but is for a lot of other people. I think it's a personality trait.
Not as a writer, but as a programmer, I can't attest that it's not just a lack of internet - it's a lack of distractions.
When I was a kid without internet or a computer, I continually got distracted from my schoolwork by a book, Legos, or just staring off into space and daydreaming.
Now, as an adult, even when I don't have access to the internet, I can get distracted by tweaking my Emacs config, playing with one of my personal programming projects, organizing my files, or playing a video game I've already downloaded.
It's not just the internet - it's distractions in general, and the thought-patterns (or personality traits) that cause us to be distracted.
However, you might have noticed a theme with my modern set of distractions - they're all on a computer, despite my library of books and my ability to stare off into space whenever I want. This reinforces the idea that technological distraction is much greater in effectiveness than (most) non-technological means.
--This dune story,
--Game of thrones (asoif) written on wordstar
--And of course dozens of writers who still use pen and paper
This is what these environments have in common: no or minimal internet to distract you