It says only that this may be what they are doing, not that it is for sure... a little late they speculate other alternative as well: _Roger says animals can actually use an oxygen-free process to produce energy from sugar, but it's far less efficient. He suspects this may be what Henneguya is doing_
It's called glycolysis, and isn't at all exotic. All animals rely on glycolysis when energy requirements outpace oxygen availability.
This little sucker obviously adapted to an environment with little available oxygen and lots of ATP. So the resource gain from losing superfluous stuff outweighed the need for aerobic metabolism.
