Matt - In case you have not seen it, I have just updated my long-standing summary on the issue behind what Robertson is saying, which I believe needs to be taken in context of Plato's Philebus. This contains my most complete list of citations on the topic.
The Full Cup Model: Pleasure, Purity, and the Limit That Answers Plato
An analysis of the Epicurean 'fullness of pleasure' model -- showing that the doctrines of painlessness, the limit of pleasure, and the terms ataraxia and…
epicurustoday.com