Ponter's idea is expressed through this analogy:
Ponter has tried to bias the example by picking something very fanciful. Let's pick something less fanciful. Suppose that God simply promises 1 ounce more gold than currently exists. Well, in that case, I think we would all recognize that God would not be challenged to fulfill that offer simply because of the present non-existence of the last ounce of gold, since God can easily make more gold. It doesn't even require omnipotence to make a finite amount of gold. So, the intuition that God cannot offer what he doesn't presently have is mistaken.
David says to his friend Paddy,
Paddy, if God were to say to me, “David, I want to offer you a green polka dotted unicorn for your next birthday, all you have to do, David, is to believe and embrace my offer, you will get a green-spotted unicorn for your birthday,” God would be thoroughly sincere in this offer.
Paddy, the Irish Leprechaun, says to David,
But that would be impossible David, because everyone knows that green spotted unicorns don’t exist in this world. God could not sincerely offer to give you something that does not exist.
Moreover, Ponter's analogy seems flawed for another reason. The gospel (in its primary sense) doesn't promise to give you a thing or object. It promises salvation from your sins. God is saying that if you trust in Christ and repent of your sins, you will be forgiven, adopted, justified, and so on.
Maybe you will say, "but what about our heavenly mansions?" Maybe you have something there! Will heaven be a ghost town of empty mansions of folks who were offered the gospel but didn't accept? Or does God actually only prepare mansions for those who trust in Christ? Intuitively, one would not expect heaven to be full of unoccupied mansions. But is that what Ponter thinks is necessary to make God's offer sincere?