The same fiduciary duty legal precedents still matter to private companies. Even private companies have shareholders (we don't have a model for companies that do not have shareholders), and the fiduciary duty of a private company is absolutely a factor in any "corporate breakup" when Founders come to disagreements, dissolve companies, et cetera.

I agree with your implication in using the word "myth" that fiduciary duty as a concern is overblown in the collective world of public corporations (the decision in Dodge v Ford was hugely problematic and its shadow haunts us all), but it's not a myth in that it doesn't exist.

Yeah, I was referring specifically to the myth quoted by the parent, "There is no promise that a company can make to its users that outweighs the fiduciary duty that obligates them to maximize profits by any means. The only defense of this is legislation and consumer choice." I don't think that people believe this about private companies.