This makes sense to me. Keep in mind this is a number over what kids whose parents do not regularly read to them. The more one reads the more exposure. More exposure improves vocabulary by tuning the brain in this critical period to the acquisition of it.
At these volumes, variety is needed to maintain novelty and stimulation. That means a personal library probably is not going to cut it unless you are wealthy. Fleur has a couple hundred books already. I expect her to have a healthy library. But, we will need to supplement with the library or hanging out in a bookstore.
Libraries also have programs for encouraging reading. A thousand books seem to be the target for the programs I see. But, I think that is the kid reading not being read to.