There was no quantification of ambient room light attempted beyond nightlight yes or no, or room lighting. There is no mention that a researcher even made any measurements of room light levels in actual households for comparison. No proposed mechanism is advanced. Classic myopia is caused when the eyeball is out of round - it is too deep or long in relation to its height. There may be an association, but that definitely does not establish cause and effect.
Nearsightedness is known to be familial.
Perhaps nearsighted parents are both more likely to use a nightlight or lamp to help themselves navigate in the night and also more likely to have nearsighted offsprings on a genetic basis. Perhaps you recall the coffee causes pancreatic cancer scare of a few years ago. The researchers were so convinced of their findings that they all stopped drinking coffee. Of course, later they had to sheepishly admit that they had been gloriously wrong - mislead by their numbers, as so many are. Examples of this sort of scientific embarassment are legion.