I assume that George is not being sarcastic here. And I agree with him.
To call oneself a Platonist today does not commit one to a 4th Century BC
worldview, or to many doctrines that Plato himself actually held. A
Platonist today would accept the existence of universals and of Forms of
some sort. But Christian philosophers who are Platonists would
emphatically argue that the universals which exist together with the Forms
exist in the mind of God.
As for Moorad's comment that we cannot know what does not exist, I think
that we can have counterfactual knowledge of a proposition which is false
(or possibly of a state of afairs which does not exist). Two examples: I
can know that (given certain assumptions such as that the laws of nature
continue uniformly) a steel ball would have hit the sidewalk below at time
t had not that freshman walked under it. If that kind of knowledge doesn't
convince you, then try this: In 1 Samuel 23, David asks God if Saul will
pursue him to Keilah, and if the people of Keilah will hand him over to
Saul. God answers "Yes" to both, so David leaves Keilah, Saul does not
pursue him there, and the citizens do hot hand him over. Certainly, since
he had God's word for it, we should agree that David had counterfactual
knowledge in this situation.
PS A few weeks ago, in a parenthesis, George said something about the
entire universe being a mathematical equation in the mind of God. That's
probably the wrong way to put it, but I would like George to elaborate.