As I understand it, life is the greatest good. Pleasure is the guide to and goal of life.
That is DeWitt's formulation, of which the second sentence seems completely accurate as what Epicurus taught. As for the first sentence, I largely agree with it, but my current view/understanding of the issue leads me to focus on it being true only in the same way that DeWitt analyses the phrase "all sensations are true" -- as a statement where you have to be very careful with the definitions of the key words.
Here I think the main issue is that term "the greatest good." "Greatest" is probably clear enough, but "greatest good" has some major ambiguities, and i am not sure that Epicurus really endorsed a concept of a "greatest good" in the way that the term was used by the other Greeks. What is "good" other than pleasure? It is pretty clear that Epicurus held nothing to be intrinsically good - worthy of choice in and of itself - other than pleasure. And there are an innumerable number of ways to experience pleasure, none of which are intrinsically "better" than others in and of themselves.
I think in part DeWitt is focusing on his observation that "pleasure has meaning only to the living" and to the resulting observation that unless we have life, no pleasure is possible, which makes life that without which there is no possibility of experiencing pleasure. But life as a condition of pleasure is different from saying that something is a guide, or even a goal.
My current viewpoint is that a "greatest good" analysis (the framework with which Torquatus starts off) is probably not an approach that Epicurus himself thought well of, and probably arises from the Epicureans feeling obligated to respond to the Stoics and Platonists. Trying to define "greatest good" too precisely probably smacks more of a Stoic / Platonic tendency to want to come up with a precise definition in words of something that is inherently impossible to express completely in words. I think that is the feel we get from what Epicurus said that Plutarch summarizes as :
Plutarch, That Epicurus actually makes a pleasant life impossible, 7, p. 1091A: Not only is the basis that they assume for the pleasurable life untrustworthy and insecure, it is quite trivial and paltry as well, inasmuch as their “thing delighted” – their good – is an escape from ills, and they say that they can conceive of no other, and indeed that our nature has no place at all in which to put its good except the place left when its evil is expelled. … Epicurus too makes a similar statement to the effect that the good is a thing that arises out of your very escape from evil and from your memory and reflection and gratitude that this has happened to you. His words are these: “That which produces a jubilation unsurpassed is the nature of good, if you apply your mind rightly and then stand firm and do not stroll about {a jibe at the Peripatetics}, prating meaninglessly about the good.”