The claim is being made that Jesus was saying (in his words recorded in John 8:58) that before Abraham was born that he was foreknown. In other words, the claim is that Jesus' existence before Abraham was simply in the foreknowledge of God, and not that Jesus actually had a prehuman existence.
Nevertheless, in John 8:58, Jesus was answering the question, "You are not yet fifty years old, and have you seen Abraham?" (John 8:57) Was Jesus simply stating that he was "foreknown" before Abraham? Hardly! Jesus was actually stating his existence before Abraham, in effect, he was saying, YES, I have seen Abraham, because I was existing before Abraham.