I'm not sure what you mean. All managers are paid to ensure better outcomes, where "better" is defined by the company (not by the engineers, except insofar as this enables the company's goals). Some managers are more effective at this, and some less. As a manager, sometimes it's better to have a cohesive team with consistent rules, even if some engineers will opt to work elsewhere as a result.
Although I am sympathetic with OP's feelings that a strict 9-5 rule is most likely unhelpful for everyone, I do think if the engineer took a confrontational tone, the manager would have no choice but to call his bluff. Any other move, and no one would take that manager seriously again. Therefore, the top answer in the original post is correct: ask nicely, and if compromise is impossible, start interviewing elsewhere. If he can really get a 40% bump, it's probably worth taking that.
I suppose "manager" is industrial aged thinking for a company with that particular culture. Good developers are self managing and self actualised and self motivated I believe. Support/facilitate is an apt role for someone to massage the backlog and keep accountability with stakeholders.
the manager would have no choice but to call his bluff.
It's exactly the same for good devs confronting a confused manager. They must call the manager's bluff, and that often (usually?) means leaving. Less experienced devs will try to push back, but it's probably as hopeless as the dev imagines it to be, and they risk just prolonging a bad situation.
Although I am sympathetic with OP's feelings that a strict 9-5 rule is most likely unhelpful for everyone, I do think if the engineer took a confrontational tone, the manager would have no choice but to call his bluff. Any other move, and no one would take that manager seriously again. Therefore, the top answer in the original post is correct: ask nicely, and if compromise is impossible, start interviewing elsewhere. If he can really get a 40% bump, it's probably worth taking that.