No, your premise is true only in a limited sense. It is not true that existence as of a certain time-frame precedes knowledge. ** The conclusion of our agreed statement (There is no winner in powerball) is that statements contrary to fact cannot be known at any point in time. (If there is no winner this week in powerball at no time** can anyone name a winner, even an omniscient God.)
This is separate from the issue of God knowing the future. The statements describing future events are not contrary-to-fact but describing events in the future that will occur.
Fore-knowledge, pre-science, pro-videntia, all mean that God is seeing all events in one view. The future events from our time-frame are not so for God, as he exists outside time.
Thus, existence is not the prerequisite to knowledge in an unqualified way. Existence precedes knowledge only in that a person within time cannot have knowledge of non-existent entities.
Bt, God is not within time and thus can have knowledge of events that happen in the future. **God knows the future because it does exist for him-- right now, as we speak. **
So God does not have knowledge of non-existent entities, as these future events currently and continually exist for him.