I think you're projecting your own personal biases and grievances into this scenario. I don't think it's a given that this was burnout related at all. You have no proof that it was, and yet you persist in treating it like a foregone conclusion.

I also think that you're wrong in making it the manager's job to police to work habits of their subordinates. I think a good manager should recognize signs of burnout in their subordinates and bring it up to try and address it, but you're arguing that the manager should impose their own idea of a healthy schedule on their subordinates rather than trusting them to know what works best for them. That strikes me as paternalistic.