At risk of deviating off track I want to call in this suggestion / illustration. I think the opening items of the Principal Doctrines are best viewed in the same way we are discussing here: they tied to particular contexts, in which they are "antidotes" to popular misconceptions which Epicurus expected his readers to confront.
He expected them to confront all sorts of misconceptions about the gods, so instead of dealing with all of them separately he gave them the key to unwinding all of them - by referring to the nature of what a "true god" would certainly be like, and not be like.
He expected them to confront all sorts of threats and promises about life after death, so instead of dealing with all of them separately, he gave them the key to unwinding all of them -- that death is the end of all sensation, and therefore NOTHING can happen after death.
And then -- and here is the current issue --
He expected them to confront all of the Platonic and other arguments about why pleasure cannot be the goal of life, most all of which are based on some version of the "pleasure is insatiable" and "it can't be the best because it has no limit, and so he gave them the key to unwinding all those "logic traps" - he pointed out that human life gives us a limit of how much we can experience, and that the very most pleasure we can experience in life is the amount that we experience when all pain has been eliminated. THAT's the context and the reason for the entire "absence of pain" discussion, and it makes perfect sense when viewed in that context, but seems absolutely inscrutable to us - because we haven't read Philebus, we don't know anything about Plato, and we've totally lost the common context that any educated person in ancient Athens would have learned from childhood!