Does it increase focus overall? I have no idea.
Quite a few times, I've been so hungry that it interfered with my thinking. I had to eat something to get my focus back. Currently sub-10% body fat and a pretty seasoned dieter, so I think I have a fairly good feeling for what's real "hunger" and what's just "craving food".
In 2 weeks I'll switch to a weekly 36 hour fast, but I have no idea how I'm going to deal with the hunger pangs affecting concentration. Maybe, as you say, it is something you can train yourself out of.
If you want a softer landing, I recommend the 16+8 fasting for a week before diving into multi day fasting. You eat at the latest at 20 (8pm), and not until 12 (12am) the next day. It will take some adapting, but it's better than the headache inducing "withdrawals" you'll feel jumping headfirst into 36h fasting.