You are comparing two different metrics: depression and anxiety per se on the one hand, and successful suicide attempts on the other. It can be true both that rates of depression are higher in girls than in boys and that rates of suicide are higher in boys and in girls.
Furthermore, while in that age group boys are more than 4 times as likely than girls to take their own lives as of now, the question is whether that proportion was the same before the advent of social media. For instance, maybe at that time boys were 10 times more likely to do it, which may support the hypothesis that social media is to blame—or maybe they were only twice likely, which would be evidence against it.
Finally, while depression and anxiety may be increasing in both boys and girls, I understand that Haidt is pointing out that these disorders are increasing faster in girls than boys.
Furthermore, while in that age group boys are more than 4 times as likely than girls to take their own lives as of now, the question is whether that proportion was the same before the advent of social media. For instance, maybe at that time boys were 10 times more likely to do it, which may support the hypothesis that social media is to blame—or maybe they were only twice likely, which would be evidence against it.
Finally, while depression and anxiety may be increasing in both boys and girls, I understand that Haidt is pointing out that these disorders are increasing faster in girls than boys.