Prose thrives in the terminal. Ice and Fire was written in WordStar, as just one popular example.
I can't find the link but I think at some point she compiled her own nano with some "helpful" feature patched out again.
For starters, there's your assumption that there is "syntax" to be highlighted. Not every text file is something written in a computer programming language.
In fact I'd put money on it, but sadly do not have any evidence to back it up.
If you have evidence to the contrary I'd be intrigued!