Interesting. I wonder if it's a newer machine with DOS installed, or if he's still using some old 386… If it's the latter I'd worry about finding replacement parts when it inevitably breaks. If you've got an ST-506 disk, you'll never get a replacement for that!
I worked in a lab where we had a computer dedicated to running a very specialized piece of equipment. The computer was a 386 running DOS. We had all of the software updates (including operating system) to upgrade everything if needed in case the computer stopped working. That was only a few years ago, and as far as I know that 386 is still running. After seeing the current software for running that piece of equipment, I understand why the lab never upgraded. The old UI was a bit clunky, but it was much easier to use.
But couldn't he just turn off spell check? I mean, the real reason is that he probably likes WordStar 4.0 more than the modern word processors, and I can totally get that, but the spell check excuse is a bit ... weak.
There are a lot of other reasons for using a single-purpose device when doing something requiring focused concentration. I think a lot of the "I read better when it's a physical printout" crowd can be won over by e-readers which only load one document at a time, for instance. It's a lot harder to read even a printed book if you have a bunch of letters and bills stacked next to it.
I can't help but think at least part of this is a bit of trolling by GRRM to his fans who (probably accurately) about his glacial writing pace.
If it ain't broke...
Well sure, I'm not saying that he shouldn't use WordStar 4.0 on DOS. If it works for him, he shouldn't have to justify it. It takes time to become comfortable with a new word processor. But if you're going to make up excuses, pointing at a feature which can be disabled by un-ticking a checkbox is kind of lame.
Sounds like he wants a text editor, not a word processor. Someone should tell him about Sublime Text.