The only real evidence they present is the Stanford study that found charter schools don't perform better than regular public schools. So their argument is tha

The only real evidence they present is the Stanford study that found charter schools don't perform better than regular public schools. So their argument is that unions can't be the problem because charter schools don't have them and are doing just as bad. But that's a straw man. Charter schools allow for better management they don't guarantee it. So in the first few decades of a charter school system there will be a lot of bad ideas and a lot of good ideas. But the bad ideas will throw off the average. But the bottom line is at least the well managed charter schools can take steps to correct problems where as regular public schools can't because of union rules. Which is why some Charter schools do dramatically better.