Ait usually works the same way as dicit. The normal Latin construction would have been ait se esse paratum. But here Horace imitated a Greek construction where verbs of saying can be followed by a nominative-and-infinitive construction. There isn't really any pronoun implied (if there were, the adjective would still be accusative). It's just understood that the subject of the infinitive is the same as that of ait, and the adjective agrees in the nominative with that subject. That construction makes the verb ait work the same way as a verb like volo. Compare: vult esse paratus = "he wants to be ready" ---> ait esse paratus = "he says to be ready" (you don't talk like that in English, of course, but it's the literal translation; it could work in English with a slightly different verb: "he claims to be ready").