I was also thinking along these lines as I read, and there have been plenty of articles posted on HN on the diminishing returns of working too long. Basically it boils down to people getting tired, and especially in creative pursuits, it helping to get away from your project and let your mind wander sometimes.

What I expected the author might say is that with many accumulated hours you become more efficient such that over time your output per hour grows, but his view seems quite different.