Welcome to the forum, Anna!
Yes, the original, literal meaning of αἴσιμος is, as you say, "appointed by the will of the gods." But, since the gods' will is always for moderation, i.e. careful "measuring" (at least in ancient Greek culture), this saying means almost the same as the famous admonitions μέτρον ἄριστον ("proper measure / moderation is best") and μηδὲν ἄγαν ("nothing too much"). In Odyssey 15.71, for example, when Menelaos says that "all things are better in moderation," he means that we must not overdo even hospitality.