At least in my (admittedly short) experience there, I can't say there was much social pressure to stay late. You had deadlines, you had to do what you had to get done, and it had to be done well. How long it took you to get there or at what times of day didn't matter very much, if at all.
Presumably he meant "impossible in a 40 hour work week," so that if you're killing yourself with an 80 hour week to meet deadlines, you're not a counterexample.
Of course, there's a prevalent opinion around here that working over 40 hours a week always decreases your productivity (and not just per hour), and the people who buy into that will likely find this definition of impossible to be silly (since as a corollary, every possible thing is achievable in 40 hour weeks).
I personally would argue that an 80 hour week isn't the same as two 40 hour weeks. It would probably be more like a 40 hour week and a 20 hour week. The end result? You get something done sooner, but you end up putting in a lot more time than you would if you just worked 40 hours a week. It's more efficient for the company because they're getting a product faster and only paying for 40 hours worth of work a week. But it's less efficient for the employee because they're putting in many more hours in total to do the same task but only getting paid as if they were working 40 hours a week.
In my experience, working long hours is only good in the short-term. Sometimes that is necessary. But it is almost never a good idea in the long-term.
I'm not going to dig up a link, but a number of people think your total productivity goes down (not just per-hour productivity, as I emphasized in my post). So if it's a strawman, it's a strawman with internet access and an HN account.
I think your personal experience matches mine - I'm more productive in my initial working hours (maybe discounting the first hour of getting in to it), and are less efficient in my later hours. But my total productivity goes up as I work more, at least up to around 80 hours/week. Whether or not that's good use of my time depends on how enjoyable the work is to me and what kind of non-work I'm coming home to that day.
I may also agree with your position on long hours being a bad idea "in the long-term", but I'd quibble with it the way it's worded. Periodically working long hours can be good for long term progress, but working long hours over an extended period of time leads to burn-out.
