The joy of resolving a philosophical doubt

To the best of my knowledge, this specific example is probably something that the author thought up, but it is entirely consistent with the limited source material. In particular I can point to one such specific example from Epicurus's "Letter to Menoeceus" in which the following is relevant.

By pleasure we mean the absence of pain in the body and of trouble in the soul. It is not an unbroken succession of drinking-bouts and of merrymaking, not sexual love, not the enjoyment of the fish and other delicacies of a luxurious table, which produce a pleasant life; it is sober reasoning, searching out the grounds of every choice and avoidance, and banishing those beliefs through which the greatest disturbances take possession of the soul.

As an example, "the 'joy' of resolving a philosophical doubt" is very much consistent with the bolded portion above.