Could it be that they go deep enough to get heat from a hot core?

I never thought about it like this, but it is a little strange that such an eco-centric author/book had a relatively shallow description of one of the main components of the ecosystem. Then again, the mystery may have been intentional. After all, we still don't understand perfectly well how many ecosystems functions, especially in the since of having a causal model sufficient to to predict responses to perturbations. I imagine at the time of writing ecosystems at large seemed even more indiscernible, so it wasn't a stretch to have some part of the many cycles involved that didn't make sense. But overall I lean toward it being kind of silly, as you say.