It's about the relationships between humans and the planetary environments. Herbert made it quite clear not just in that interview but throughout the first three novels at least that he was interested in the structures that affect things such as the development of religious, political, and social systems. Those are parts of the branch of ecology called human ecology.



Don't have to have a directly-stated "message" for there to be a theme; since politics, religion and power fall under the umbrella of (human) ecology in most definitions, it's hard to see what your complaint is, unless it's just a semantic quibbling.






Here's a set of definitions of ecology that places that Herbert comment in a better context:




Yes, it is one theme. Another would be how political/social/religious institutions develop and how they are related to their environs and how these institutions shape and are shaped by what is around them. I seem to recall an observation in one of the first three books (one that I've seen echoed in several other places) that it is no surprise that the three dominant monotheistic religions on this planet today developed in steppe or desert-like environments. Several social practices are also affected by the surroundings and ways political power is structured (as Leto II and the Preacher discuss in Children of Dune) is based on practical consequences of physical surroundings.

I just see those causal relationships being part of an ecology, same as I would view modern-day political parties as being part of an ecological system. And those ecological structures and how they affect the narrative history seems to be a theme in the Herbert novels.