We **could** argue with the sources--Philodemus, Polystratus, even Epicurus--, but I would prefer if people don't do it until they've made a good will attempt to get what was being said. I'm sure Philodemus understood what was wrong with idealism, and I'm sure he wasn't trying to teach idealism, because he was immersed in a society that was plagued by Platonism and understood the issues.
There's another quote from On Piety where Philodemus talks about how piety helps us to cultivate pleasant "psycho-somatic dispositions". I think this passage is key and should be evaluated next to all the other passages, because it implies that the exercises in piety were meant to have effects on the health of the body AND the mind (psycho - somatic).
So considering how scarce our sources are, rather than dismiss this discourse, it should fall to us to try to reconstruct these ideas, and here is a MATERIALIST theory of piety, one which constitutes a unique contribution that EP makes to ethics, and also one that can be verified against research on the healing effects of chanting and other pious practices. Here is also a chance to show how the Canon is used. We appeal to evidence and check he doctrine against studies available. In my book I cited studies by Marian Diamond which documented and quantified the health benefits of chanting, including lowering blood pressure and heart rate. THIS type of thing vindicates the assertion that piety is meant to have a psycho-somatic effect. And again, this is therefore NOT idealism, it's a way of looking at religious practices from a purely materialist perspective.
Similarly with "imitating their blessedness" and "making oneself harmless", etc. Piety is meant to be an ethical exercise that helps to cultivate a certain kind of disposition (diathesis, a word we find in Philodemus and also in Diogenes of Oenoanda) and character, just like we exercise our body.
Also, what's being said here is not "do not harm, ever". That is NOT the point. What's being said here is "these practices will make you of a certain, harmless disposition". The tacit idea is that this is a disposition that is advantageous or pleasant or desirable, particularly among friends or people who engage in pious acts together. Just like when we associate with certain kinds of people who help us develop a good character, similarly with these practices.