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).
