Jimmy -- thanks for posing these important questions. The blog does not call for providing "leadership" training everywhere -- rather, it points to emerging evidence that providing pedagogical support for teachers is crucial. We're trying to shed light on the reality that, currently, many school principals are not equipped to provide teachers with such support. As of now, I have not come across rigorous evidence on the sort of training you describe; however, I'm happy to learn.
Regarding the question on other problems principals, schools, and society face, I think these should inform, but not prevent us from thinking of ways to utilize principals to improve learning. On a slightly lighter note, I think we'd both agree training people is possible, considering the time and efforts spent pursuing a PhD. If this wasn't the case, it's unlikely we would have chosen to pursue a graduate degree. Of course, this is not to say we are all equally good at what we do or we all were born with the same innate ability. At the end of the day even Messi wakes up early every morning to train and grow as a player.