I think the joke hinges on equivocation of the word "intelligent". Taboo "intelligent", use "sapient" and "clever" for the two meanings, and you get: "Sometimes I think the surest sign that clever life exists elsewhere in the universe is that no sapient life has tried to contact us." Or, put more accurately, "the fact that no sapient life has contacted us is evidence that, if sapient life exists elsewhere in the universe, it's probably also clever".