Reminds me of the graphs that Julie mentioned, where pleasure over time can only reach a "set ceiling", and that no matter the amount of time in any given example, continues to be a non-factor. I think a part of this misunderstanding comes from the Epicurean counter-argument against fearing death, as mentioned in his letter to Menoeceus: "And therefore a right understanding that death is nothing to us makes the mortality of life enjoyable, not because it adds to it an infinite span of time, but because it takes away the craving for immortality.". Where now the standard for Epicurean philosophy is; live life pleasurably for when you die you cease to exist, it only makes perfect sense to want to extend that pleasure for the longest amount of time as possible.
But of course, in many other sources such as PD 19, and later in that same letter as you quote, Epicurus does not get hung up on pleasure with any sort of extenuating circumstance or modifier.