Well, assuming that in this world there are equal and opposite reactions to everything, war must therefore be the opposite of peace. If that is so, and also assuming that Hobbes knew this and also that nature results from learned habits, then he would have believed that because war was ingrained into the human psyche, that it must derive from some place other than nature.
Observing that plants are not warring in nature, then he may've concluded that man must be warring within himself constantly between the good and evil within his soul. This internal conflict often finds outlets from the soul through actions, which in turn influence others to struggle and choose between good and evil.
That being the case, Hobbes would have perceived that this ingrained behavior was not the original state of nature, and that Humanity must have departed from some original state of goodness. (I.E. the fall of mankind in the Garden of Eden).
However, upon noting the controversial and complex substance of the argument, he may've thought it better to go along with the theory that man in nature, is war.