If you've been writing a lot of software, I would expect you to be good at actually putting the characters into a file and moving them around therein. I would be very suspicious of someone who claimed to be a master carpenter but couldn't drive a nail cleanly. Muscle memory is a real thing, and the mechanics of a task matter. What is the point of all our fetishization of editors and keyboards and such if not to smooth the transition between brain and paper?
This isn't even always a matter of candidates operating in an unfamiliar environment. Even on their own personal computers, in the editor they presumably use for work, I see a lot of flailing.