Epicurus didn't formulate this argument, it's a paraphrasing of something put in his mouth by Lactantius, a Christian author who lived in III-IV century AD and who used him as a character in a Socratic dialogue in his treatise On the Anger of God. No such argument can be found of any of Epicurus' surviving works. What we do have of his writing shows that he did believe gods existed and had a strong sense of divine justice, but he had some unusual views about them.

The thing is, ancient sourcing is more concerned with portraying a sort of equivalent moral tone than it is about accurately representing someone's writings or words. If you look into Thucydides, he'll quite openly say that he made up most of the speeches, but kept them in the sort of style and tone of the speakers. There's a longstanding suspicion in academia that a lot of what "Socrates" said in the various Socratic dialogues is just Plato attributing his own ideas to Socrates.

It wouldn't even have been viewed as dishonest, a kind of "I bet that if Epicurus was here talking about the concept, he'd say XYZ, so I'll just put those words in the mouth of Epicurus and call it a day".