Is that a result of his metabolism adjusting upward, as opposed to simply burning lots of energy on activity and probably having an unusually high metabolism in general?

With Michael Phelps it's tricky. There are actually three parts here. Part of his energy balance goes just to do the physical movements of swimming, part goes to increased heat production as he spends hours per day in a cold-water pool, and part is just high resting metabolism.

Eating consistently at a certain level without somehow tracking calorie intake seems exceptionally tricky

Depends on how your food is structured. But if you're counting calories already, you can just continue to do this, just don't have a fixed level in mind. E.g. last week you were eating, say, 1,700 cal/day and lost zero pounds, so this week reduce to 1,500. Don't pay attention to absolute numbers, all you need to know is whether to shift up or down.