I think it would provide very little productivity boost if any (is anybody’s work seriously bottlenecked by the time it takes to find the navigation keys? What are they doing?). But also, I can’t think of any reason not to use the keys that you rest your fingers on for navigation. I mean, why not?
A good argument could be made that vi is all about the typing speed boost you get from not using a meta key like control (or later alt or fn). The specific keys didn't matter, although having them near the home row is helpful.
Vi has the same model input model as teco/xtec and shares their key press economy. Emacs which started life as a teco macros went the other way. Decades ago I tried both and abandoned Emacs when the finger in charge of the control key started to ache. It was not just slower, it was painfully slower.
The article does not draw any conclusions on whether hjkl navigation leads to a productivity boost or not. I’ll just say, it certainly feels cozy to have navigation where your right hand is meant to sit according to touch-typing canon. Maybe that’s why they put the arrows there in those legacy keyboards in the first place.
Then jkl; would make more sense, since it wouldn't require moving one's right hand from the home keys on a QWERTY keyboard. If I'm going to shift right anyway U for up, H for left, J for down, and K for right makes more sense (like arrow keys).
