You don’t need negative marginal productivity after the 55th hour to see a negative impact of more than 55 worked hours a week. Things like exhaustion will affect you all the time and decrease overall productivity. So sure, you might still do some useful work after 55, but over the course of the week you’d do still less than if you worked 40 hours. There are caveats and exceptions as usual, but it is not as ridiculous as you make it sound.