Isn’t that just a symptom of ADHD? I would say the advice still holds. Movement is just to stimulate your mind to retain focus, but in this case we don’t want stimulation, so you’ll still be able to (with will power) remain still in bed.

No, it’s not (see e.g. https://www.nhs.uk/conditions/restless-legs-syndrome/).

It’s indeed difficult to lie still when you have RLS, but general sleep hygiene helps with it.

It seems for my wife it may have been low iron, second-hand anecdotal reports on low-iron forums only is my basis, sorry.

I had RLS pretty bad, and that makes it impossible to lay still in bed. I "cured" the RLS somehow. It was probably either, or a combination of, going keto and/or giving up a lifestyle of chronic cardio exercise. I've been RLS free for over a decade now. Lying still in bed is possible now, though still not super easy for me, but at least RLS isn't part of the problem.