And if they are insane, in what constitutes sanity?
From my readings, I interpret this: "insanity" is the rejection of reality, and "sanity" is observance of reality. An "insane" person is full of pseudodoxies, false beliefs and opinions. A "sane" person makes "true" statements which properly correspond with the physical reality around them.
One such physical reality in post-Alexandrian Greece was the exchange of foreign forms of piety, and the need to explain the existence of these foreign, yet recognizably spiritual institutions. Surely something natural exists at the source of this seemingly-universal sense of piety.
Epíkouros calls this the divine nature. (Whether or not that corresponds with theoretical super-humans is up to you, but at least this much is true: a natural phenomena is responsible for the evolution of religion). Philódēmos records this as something like visions during dreams.
I believe that Epicureans dismissed atheists as being "insane" because they interpret atheists to be "rejecting the existence of the naturally-occuring, universal, awe-inspiring dream visions", as well as "reducing the practice of piety to a delusional narrative that has no basis in physics."
In that regard, I think he is making more of an anthropological observation than a theological proposition. I ... wonder if it is possible that they were, sort of, talking passed each other? They were both anti-creationist, anti-interventionist, anti-superstition, anti-metaphysics, etc.
I get the feeling that ancient atheists didn't buy into Epíkouros' definition of "the gods". I think, maybe, the interpretation of Epicurean Philosophy by ancient atheists was similar to ancient Christians: 'god must be seen as a supernatural super-being' is what they're observing.