Yes, that's true. Muilenburg likely would have to convince a cadre of other executives and probably board members that the plane wasn't safe and shouldn't be released, which wouldn't be easy. But what's worse...a bunch of sunk costs in a flawed design, or a bunch of sunk costs in a flawed design that crashed, killed a bunch of people, and now nobody wants to buy your plane?
You'd think that anybody, even a "bean counter," would know that ultimately the price of deaths and the awful PR is way higher than being late with your plane relative to a competitor. If Boeing really thought that plane would crash due to a half-ass design, or that there was a good chance it would, there's no way they would have released it as it was. They knew they rushed the design, but they probably genuinely thought it was safe. I know it's so easy to frame large corporations as evil and only caring about money, but since they do care about money, there's no incentive to knowingly release a dangerous plane.