It's not a myth, per se, that managers have a fiduciary duty to shareholders. They clearly do have such a duty.

The myth comes in where people believe that means the burden of proof is such that they have to blindly serve short term financial interests of shareholders. There is no such rule at all, in fact it's quite the opposite:

https://en.wikipedia.org/wiki/Business_judgment_rule

The business judgment rule is a case law-derived doctrine in corporations law that courts defer to the business judgment of corporate executives. It is rooted in the principle that the "directors of a corporation... are clothed with [the] presumption, which the law accords to them, of being [motivated] in their conduct by a bona fide regard for the interests of the corporation whose affairs the stockholders have committed to their charge".[1] The rule exists in some form in most common law countries, including the United States,[1] Canada,[2] England and Wales,[3] and Australia.[4]

To challenge the actions of a corporation's board of directors, a plaintiff assumes "the burden of providing evidence that directors, in reaching their challenged decision, breached any one of the triads of their fiduciary duty — good faith, loyalty, or due care".[5] Failing to do so, a plaintiff "is not entitled to any remedy unless the transaction constitutes waste... [that is,] the exchange was so one-sided that no business person of ordinary, sound judgment could conclude that the corporation has received adequate consideration".

As your own citation points out, that duty belongs properly to the board of directors (the CEO is often also Chairman of the Board of Directors, and many of the CEO's initiatives may be either directed by the Board or require its approval, so in those senses the CEO's actions may be subject to the Board's duty, but the duty is the Board's, not the CEO's.)

The defendant in this case was 60% owner of the company and as such can be presumed to be one and the same as the board of directors for purposes of this discussion.

With that said, the basic principle is that managers are considered to have a presumption of good faith in their business judgement with regards to disputes with shareholders over business strategy.

Fair enough. The problem is that in the context of this particular article the CEO could easily make the case that he's doing just that so invoking this as a counter argument against what the CEO did makes no sense.

This particular argument is only in regards to Dan Price's pay as CEO and his use of company credit card. This suit was filed before his $70,000 pledge and according to his brother may have been done so in retaliation to the lawsuit to deplete the company of positive revenue which hurts the brother who filed the lawsuit

Elegant!