I think this is not one of DeWitts best explained positions, and I would agree with it only after caveats that "good" like a lot of other words has multiple meanings.
In this sense, I think his best line is to the effect that pleasure and pain have meaning only to the living, and that therefore being alive is a prerequisite to experiencing pleasure. That's in his explanation somewhere in a place you did not quote.
As he states in the part that you did quote, pleasure is the telos and DeWitt is distinguishing between that and "greatest good.". It seems to me he is stressing the proper differences between the words goal and guide as well.
Ultimately you should also read DeWitts article he entitled the summum bonum fallacy. He is to some extent calling into question, as does Epicurus in his criticism that DeWitt quotes about unsurpassable joy, of the issue of thinking that you are doing something worthwhile by focusing conceptually on defining "the greatest good.". Nature gives pleasure and pain as the feeling / basis for deciding what to choose and what to avoid. Nature does not implant in us a detailed reasoned definition of a "greatest good" nor do we create anything real - anything outside our own minds - by developing elaborate definitions of it.