>>For the typing condition, participants typed the same words on a keyboard using only their right index finger.

>The study might have been better if the types used a full keyboard with both hands

Agreed. This reminds me of solving Rubik's cubes, where solvers employ "fingertricks" like using index finger followed by middle finger to do a U2 (two turns of the "up" face) more quickly. Similarly people often minimize "regrips" and "cube rotations" (changing which side is facing you) to improve efficiency and get times down. What it also does is result in a more specific and consistent set of motions that I think are easier to remember and execute. People will also talk about algorithms being "in their hands" to differentiate between being able to spew out the notation vs actually doing it. I've found that for several algorithms I've memorized, if I don't do them at a natural (fast) speed and the usual positioning, I can forget the moves and lose track of what I'm doing. If I had to do every turn using my whole right hand, slowly, I suspect I would struggle a lot.