I wonder if there is selection bias with them all being military candidates.
Was thinking the same. But I'm not sure exactly in what it can be biased. The only thing that I thought of is that maybe the chareidim that they are talking about aren't so chareidi. But I don't know how the draft system works. Or that the people they counted in their study were more physically fit.
If anything what I was basing my numbers is also biased because it was mainly based on what I saw in my yeshiva where everybody is learning all day.
What you do see is that it's reversible. If it can get bad from always looking at close things, it can get good from looking at far things (without glasses).
I personally have tried this out plenty of times. After wearing glasses for a while I take them off and my vision is blurry. I don't wear it for a couple of hours and my vision is much less blurry.