Sedley's argument is that Epicurus was not a strict reductionist: he did not say that your feelings of pleasure and pain, your lived experience, your psychological states are "mere illusions" that dissolve into atomic physics if you look closely enough. The qualities of compound things — including the pleasure and pain we feel — are real, not eliminable, and must be understood at their own level. That is philosophically powerful ammunition against the modern dismissal of Epicurean ethics as "merely subjective" or "just brain chemistry."
Please explain how: "That is philosophically powerful ammunition against the modern dismissal ....as 'merely subjective'". Doesn't each person experience those things based on their own subjective physical and mental state?
Also, please explain how "just brain chemistry" varies from the Epicurus' methods and conclusions exploring those human experiences. I have trouble seeing it as a dismissal of Epicurus. The question of chemistry affecting free will to some degree Is being examined and tested. Therefore, the possibilities of chemistry partly explaining free will seems consistent with Eplicurus' methodology of reductionistic and antireductionistic logic.