In addition to that very good point about controlling for socioeconomic factors, the article says a single museum or concert per year makes a difference. Most cities have free community concerts (some even have free opera performances!) and museums that are either free, pay-what-you-want, or at least have specific days/times during which they are free or at a significantly reduced cost. Many libraries (which are free) provide free museum passes to card holders. In fact, the article quotes a museum worker who works at a free art museum in Baltimore.
If you actually read the article you would also read that educators are excited about this study because it provides evidence that the arts should be made more accessible financially - by restoring arts programs in the public schools, for example.