One can agree with all the positives mentioned in the article and your points as well but still question if there is an _even better_ investment than libraries that will result in the most good.
Even Jeff Bezos' wealth is finite so spending it is a zero-sum game. Donating to public libraries means money not donated to researchers needing $1m to find cures/antibiotics that would benefit all of humanity more than upgrading libraries. (Or whatever other recipient you can imagine that could have a greater multiplier effect of that money than libraries.)
Maybe libraries are the best use of his donation money. Maybe not.