Every language in co-NP can indeed be reduced in polynomial time to an instance of the formula ∀x P(x), where P is a Boolean formula with a single bound variable and no free variables. The definition of a language being in co-NP hinges on the existence of such an instance, along with the requirement that the predicate is computable in polynomial time. This highlights the relationship between co-NP languages and their polynomial-time reductions, emphasizing the computational complexity involved in verifying the properties of these languages.