CTO and lead developer on a focused project are very different jobs - my guess would be he relaxes from the very strategy and soft-skill heavy day job by diving into a challenging problem that keeps his dev chops up and lets him focus on a finite problem.
No idea on Evan Wallace's perspective but there is a possibility that it isn't seen as work. It would like be me solving sudoku or some of my friends building LEGOs or solving extremely hard jigzaw puzzles
