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 was 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. Another study 2 was published in Nature in the early part of 2000 which found contrary results to the first study, namely that the risk of nearsightedness is not higher in children who sleep in rooms with night lights. Currently available data simply is not convincing that parents should change their practices.