This claim is highly philosophical the way you present it whole the discussion on the all knowledge of God is more of a spiritual one rather than philosophical. The err of open theism is taking humanistic approach to explain logically how God can (1) know the future but (2) yet allow free will. The humanistic philosophical approach takes one part of the Bible and says if God can do this he should not be able to do also at the same time that (from other parts of the Bible) because this would contradict human logic. Too bad! God does not concern Himself with being the proper subject of human logic. The spiritual approach says simply Yes He can! God can all. God knows all. God is in all and for all!