With Epicurus, what God exactly is he referring to? Christianity would come a few hundred years later, leaving Judaism with the only monotheistic God. Classical Greek gods were most definitely malevolent and/or unable, but that quote is singular, so I am led to believe that he is referring to the concept of any sort of divine being?
You act like Monotheism is new. While the classical greeks had a polytheistic religion, the intellectuals and philosophers generally didn't practice such things, and in addition, there were a variety of religions that cropped up around the Mediterranean for ages, including Monotheistic ones. Think Persian Zoroastrianism. Greece and Classical Persia mingled a lot, particularly under the conquests of Alexander the Great. In any case, Epicurus was not arguing that there is no God. Everything scholars know about him indicates instead that his arguments were illustrations that, any Gods that do exist simply do not care about us, as they by nature are far too alien and advanced for human comprehension.
He felt that it was no accident that the morality of the Gods was always the morality of the majority of the people who believe in them at any given time, shifting to accommodate the people. His argument was against any kind of god that is so conveniently similar to us.