But isn't this article saying the exact opposite of this? It may be counter-intuitive, but every serious study I've seen regarding the amount that people work consistently says that 40 hours (or less) is ideal, and exceeding that for an extended duration (where "extended" is a very small amount of time like a few weeks) leads to negative productivity.

It seems like there's often a lot of cognitive dissonance going on in these threads - when we're talking about managers or people other than us telling us to work > 40 hours a week, it's widely accepted as counterproductive, but when we do it ourselves, we ignore the data and revert back to platitudes about pushing yourself harder and not being complacent.

The GP post saiys he hated his 40-hour job, so we can assume he wasn't at his most productive there (it happens to me too).

Those studies might have involved people doing one job for all those hours, while the grandparent worked two jobs. Depending on how different they were, the variety might have been enough to offset some of the fatigue of working up to 80 hours a week in total.