Even at 10 minutes per answer that is roughly 3.7 hours a day or 26 hours a week. That’s more than half of a full 40 hour work week or like 23% of your waking hours each day!

That’s a non trivial amount of time!

I'm willing to bet this guy is so good at his craft that he bangs out quality answers in less than 10 min per question. That'd actually be a fun side project: for a given SO user, compute their average time between each of their answers and the corresponding question.

You are assuming that he answers questions every day. He could easily be spending weekends on SO and some evenings. It doesn't mean he's using work time to do this.