Here is the argument against "essences" made by Frances Wright in A Few Days in Athens, Chapter 15. I would apply it to Epicurus' view of pleasure -- pleasure does not exist apart from the normal experiences of human life:
“What is in a substance cannot be separate from it. And is not all matter a compound of qualities? Hardness, extension, form, color, motion, rest — take away all these, and where is matter? To conceive of mind independent of matter, is as if we should conceive of color independent of a substance colored: What is form, if not a body of a particular shape? What is thought, if not something which thinks? Destroy the substance, and you destroy its properties; and so equally — destroy the properties, and you destroy the substance. To suppose the possibility of retaining the one, without the other, is an evident absurdity.”
I would say that the error OKeefe and others are committing is that they are attempting to conceive of a pleasure apart from experience, just as if color could be separated from things that are colored. And this is a huge error, because if Epicurus' intent in describing "absence of pain" is just to describe the experience of ordinary pleasures without any mixture of pain, then their whole argument toward asceticism is out the window - because the experience of living without pain means the normal day to life mixture of pleasures, with no mixture of pain.
Or, as Cicero described it: "a life of tranquility crammed full of pleaures"
Cicero, In Defense of Publius Sestius 10.23: “He {Publius Clodius} praised those most who are said to be above all others the teachers and eulogists of pleasure {the Epicureans}. … He added that these same men were quite right in saying that the wise do everything for their own interests; that no sane man should engage in public affairs; that nothing was preferable to a life of tranquility crammed full of pleasures. But those who said that men should aim at an honorable position, should consult the public interest, should think of duty throughout life not of self-interest, should face danger for their country, receive wounds, welcome death – these he called visionaries and madmen.” Note: Here is a link to Perseus where the Latin and translation of this can be compared. The Latin is: “nihil esse praestabilius otiosa vita, plena et conferta voluptatibus.” See also here for word translations.
Or as Torquatus described it in On Ends: "living in the continuous enjoyment of numerous and vivid pleasures alike of body and of mind, undisturbed either by the presence or by the prospect of pain"
"The truth of the position that pleasure is the ultimate good will most readily appear from the following illustration. Let us imagine a man living in the continuous enjoyment of numerous and vivid pleasures alike of body and of mind, undisturbed either by the presence or by the prospect of pain: what possible state of existence could we describe as being more excellent or more desirable? One so situated must possess in the first place a strength of mind that is proof against all fear of death or of pain; he will know that death means complete unconsciousness, and that pain is generally light if long and short if strong, so that its intensity is compensated by brief duration and its continuance by diminishing severity. Let such a man moreover have no dread of any supernatural power; let him never suffer the pleasures of the past to fade away, but constantly renew their enjoyment in recollection, and his lot will be one which will not admit of further improvement."