The objection to the Free Will Defense that the OP presents is similar to what J. L. Mackie presents and Alvin Plantinga quotes (page 32). Plantinga summarizes it as follows: (page 33)
Surely there are possible worlds that contain moral good but no moral evil. But
God, if He is omnipotent, can create any possible world He chooses. So
it is not possible, contrary to the Free Will Defense, both that God is
omnipotent and that He could create a world containing moral good
only by creating one containing moral evil. If He is omnipotent, the only
limitations of His power are logical limitations; in which case there are
no possible worlds He could not have created.
After Plantinga discusses what possible worlds mean, he gives two examples of possible worlds that God could not actualize even if omnipotent without contradiction.
- There are possible worlds in which God does not exist. God could not actualize these: (page 39):
Now if God is not a necessary being (and many, perhaps most, theists think that He
is not), then clearly enough there will be many possible worlds He could
not have actualized-all those, for example, in which He does not exist.
Clearly, God could not have created a world in which He doesn't even
- If there are free creatures then God cannot actualize a possible world that goes contrary to the choices of these creatures: (page 44)
If we consider a world in which S' obtains and in which Maurice freely chooses oatmeal at time t, we see that whether or not it is within God's power to actualize it depends upon what Maurice would do if he were free in a certain situation. Accordingly, there are any number of possible worlds such that it is partly up to Maurice whether or not God can actualize them.
The OP asks:
Is it fair to say, if we take omnipotence to mean: "God can do everything which is logically possible", that he is accepting that God is not in fact omnipotent?
Plantinga calls the belief that God's omnipotence implies that God can do everything which is logically possible to be Leibniz' Lapse. (page 44)
We now see that this contention-call it "Leibniz' Lapse"-is a mistake. The atheologian is right in holding that there are many possible worlds containing moral good but no moral evil; his mistake lies in endorsing Leibniz' Lapse. So one of his premises-that God, if omnipotent, could have actualized just any world He pleased-is false.
Mackie, J. L. (1955). Evil and omnipotence. Mind, 64(254), 200-212.
Plantinga, A. (1977). God, freedom, and evil. Wm. B. Eerdmans Publishing.