Boys also had an increase in self-reported mental illness, so your assumption is wrong. The article is claiming that the increase is larger for girls than boys.

Boys also had an increase in self-reported mental illness, so your assumption is wrong. The article is claiming that the increase is larger for girls than boys. However there could be reasons that it's now more acceptable for girls to report mental illness and not for boys, so it doesn't really resolve the sex comparison issue. BTW the chart you linked shows that we had higher rates in the 1980s than now. It makes the recent increase look like it's just part of a long-term cyclical trend.