I think an MBA will help you be a better manager if you want to be, because at least there's some clue about what a "good" manager should look like.
I've met sooo many bad managers. In most cases they were bad because they didn't really know what they were doing and they felt they couldn't look "weak" or not be in charge. There's always the urge to authoritarian leadership because it's the default (for some reason).
If the MBA course is any good, it will at least have exposed such a person to other leadership styles and some management theory. They might reject it, of course, but at least they'll know it.
So, yeah, I don't think having an MBA makes you a better manager or leader automatically. But I think a manager who wants to get better could be helped by doing an MBA.
Of course, there are lots of managers who are convinced they don't need to get better, and so won't/can't be helped. And there are lots of people who get MBA's because it's a ticket to promotion and don't really care about the learning.
I think even that part is disputed; we really don't know what a good manager looks like. Some of what's taught in MBA classes now is the opposite of what was previously taught, and there seem to be as many failure stories as success stories. I do take your point that maybe just thinking about anything other than the authoritarian mode is enough to raise a manager above the average, which is distressingly plausible.