St. Thomas points out that this argument…
If God knows the future determinately, the future must exist determinately
And God knows, etc.
Therefore the future is determinate.
…is identical to this argument:
If Sioux Falls is on the map, it is a small dot
But it is on the map.
Therefore, Sioux Falls is a small dot.
In other words, you can’t take God’s knowledge of the future as proving it is determinate in reality any more than you can take the mapability of Sioux Falls as proving that the whole town is, in reality, a small dot.
We can make the opposition sharper by pointing out the difference between understanding an arpeggio and a real arpeggio: understanding grasps all at once something that, if it existed all at once, would not be an arpeggio but a chord.