I'm not sure if I'm missing something (perhaps the joke, as it were) but the Calvin & Hobbes strip seems to be entirely about having a sense of humour, rather than a theory of comedy.
The physiological manifestation of the humor response is also explained by evolutionary fitness when you consider it as countering the physiological reaction to the perception of stressful ambiguity.
Also I don't think the comics final answer is satisfactory, because you could definitely respond to absurdity, by calling out why something is absurd, you don't have to laugh at it.
So why do we have a sense of humor in the first place?
The ancients eventually came up with the whole God thing to explain all confusion away but i have a feeling that happened after the evolutionary push to not kill yourself the first time you see lightening