Ask HN: How do you deal with a manager who knows the estimates better than Devs?

In one case the manager is my superior, in another it is my superior's yes-man. They have no real background in IT but are quite handy with computers - which probably contributes to the problem. They would say things like 'I could hack that website myself in Dreamweaver in 2 days' ignoring things like responsiveness, cross browser compatibility, all teh edge cases, connection to backend, QA, deployment etc. Obviously they are bad managers and they are embarrassing themselves in front of the developers. I could easily ridicule them and hand over my notice but I am curious how would a great product manager handle such situation.