The insight is that God's freedom is not that of "indetermination," which is the sort we have in via and, to a lesser extent, even in patria. That's because
If we consider the indetermination of the freedom so far as it does not possess some determinate good, then freedom is not a perfection or a good. The lack of good is not a good. So far as we take freedom in this way, we don’t call God free; and so far as freedom is taken as a perfection, and therefore said of God, we throw out the idea of indetermination-in-the-sense-of lacking-good and keep only the more central perfection (say, self- possession, or being the Lord of ones action.) We might even keep the idea of indetermination so far as we mean that God’s action is not forced by another, or so far as he is responsible for it.Quite so. But orthodox theology affirms that God is free in some sense. So Chastek says:
Again, the divine freedom, so far as there is a thing, cannot be defined without bringing in the notion of creation. The Son, for example, does not proceed from the Father’s will, but if this is the case, the divine freedom cannot be defined without relation to the imperfection of creation, and divine freedom is not taken as an absolute perfection, as though the possibility of freedom would remain if the imperfect (that is, creation) were not possible.Now God needn't have created, yet would have been fully free if he had not. Chastek knows that, which why he says that divine freedom entails only the "possibility" of "imperfection,"—a possibility which, given God's absolute perfection, is logically equivalent to the possibility of creation. So God is free just in case he can produce something imperfect, i.e., not fully determined, whether or not he actually does so. And of course, the fact that he has actually done so doesn't make him any more, or less, free.