I'm curious if this is actually conflating homeless people who spend time in the library as one of their only safe places, rather than actual library patrons.
Looking at the income breakdown at the Gallop link that throwawaylolx posted, it may well be an influence, although conflate seems a bit stronger than I'd put it.
More an effect of income inequality. Plenty of homeless in the high income inequality downtown library, but there's more people visiting in the higher income equality suburbs which swamp that first effect out when averaged nationwide.
But in any case, the conclusion in the title of the article is wrong and not supported by the results.