No, it isn't assuming submaximal pleasure is pain; submaximal pleasures just can be shown to require suffering. That falls out of poking various forms of positive experience and examining them closely. For examples: Humor involves a butt of the joke, which is suffering. Minor, brief suffering, but still. Sexual pleasure is intertwined with anticipatory desire, which is unpleasant and, in many situations, slightly painful. Satisfaction inherently requires a period of dissatisfaction preceding it, or it's meaningless; dissatisfaction is another form of (usually-)minor suffering, and rarely a brief one. Fun is difficult to distinguish from pleasure with any hard boundary, but most boundaries would say that it involves some form of contest or struggle, which requires the frustration of not having succeeded yet, and the potential for failure.