You are surely correct about this. But another interesting question still remains: suppose the cyclist and swimmer go out for a moderately paced bike ride together. Maybe the cyclist finds it very easy and the swimmer is a bit more challenged but still nothing too crazy. Will they have burned the same number of calories?

The same amount of energy was delivered to the pedals. But if the swimmer's heart had to pump blood through extra arm muscles, that is energy expended that doesn't go to the pedals.

There is still the question, too, of efficient pedal spinning. Maybe the swimmer had trained in biking some years back so still has a good cycling form. Or maybe this cyclist hasn't cycled much at all and so is perhaps pushing down too much on the back, upward moving pedal, thereby doing muscular work that actually reduces the power to the crank.

Anyway this stuff is all might speculative! I just happen to love such explorations of energy pathways!

These days one of the books I am reading is The Pleasure Trap by Lisle and Goldhammer. The basic idea is quite similar to that of Good Calories, Bad Calories by Taubes. It is impractical to try to manage weight by calorie counting. Our bodies have finely tuning feedback systems to regulate food intake. What throws those mechanisms off is the crazy food we eat, combinations that are very unlike those our bodies have evolved to work with. What works, so these authors say, is to get back to more natural foods and food combinations. I fear I cannot really confirm this theory. My fat percentage is something like 27% and I expect chocolate bars have something to do with that. And fig bars. Yum!

One place I would expect this sort of calorie computation might be useful is for planning nutrition on long rides. You don't want to run out of snacks out in the middle of nowhere on a double century... but you don't want to lug too many extra snacks along either! If your trained and experienced double century rider friend carries along X calories of snacks, and you are just getting into cycling and pushing your limits, should you just bring along the same X calories of snacks or maybe anticipate some inefficiency so maybe 1.2 * X might be advisable?