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.

The point here is that if you know you're stuck at the worksite "until it's done", you'll work at a more manageable pace.

BUT if you know your workday is exactly 8 hours with a 30 minute lunch in the middle, you'll pace yourself differently.

The difference between two equal people doing the same thing, one working until they drop and the other working 8 hours and leaving, isn't big enough to warrant the longer hours worked in the long run.

The one working longer hours might get more work done for a day, maybe a week or two. But after months of work the first person is burned out and the second one is still going strong.