The poem first establishes a boast, then replies with an apparently off-topic question. The shift supplies the comic surface, while the actual purpose is to belittle ostentatious boasting.
Were the reply an entirely innocent joke, the intended criticism would disappear. Conversely, the direct statement “your boast is worthless” would be earnest rebuke without this humorous form.
Recognizing the difference between surface manner and communicative purpose is essential to identifying the figure.