I don't applaud that. I think that he missed one important step in the design of a text editor... There's premature optimisation, as in:

"I'll use a nice, buffered storage split in chunks that can be written back to disk to save memory... Even if it's going to be used for a field that accepts max 1KB of text."

And there's proper design:

"I'm writing a program which main/only purpose is to operate on text efficiently. That means I'd better create a good file/text abstraction up front, because otherwise I'll have to rip the whole thing apart and reassemble once people start using it for something longer than 1MB"

There's another haskell editor with vi-keymap and command-mode available - Yi. I think they're using the rope concept to deal with big files.