That sentence is an oxymoron.
Destiny, by definition, is preordained, and by this same definition, cannot be changed.
Hence, you prove that there is free will. If there was such a thing as destiny, then the people of Nineveh could not have repented and God could not have changed His plan to destroy them. There would have been no chance of God changing his mind.
Whatever occurs has been preordained by God and as such, is unchangeable from His decree.
Including the fact that you or I sin? So God says Doc E and Cronos would sin in this way and that and Hitler in this way and that and because of these sins, some will go to Heck. So, God says -- ok, let's plan this person's life to sin and go to heck.
Or else the world is open-ended and up for grabs with God watching from the sidelines while He ponders our next steps right along with us.
"Who is there who speaks and it comes to pass, unless the Lord has commanded it?" -- Lamentations 3:37
No, the fact that God has not already decided who will sin and go to heck and who won't does not render God impotent, rather, forcing God to be a slave to a Plan is rendering God impotent. God is not watching from the sidelines -- He can choose to act, as He did by chasing Jonah or by parting the waters of the Red Sea. He chooses not to control us like puppets because that is His will.
You guys keep pushing this "according to Protestants, God is a slave to His plan so He cannot be omnipotent" thing. Of course we're not saying that. Look at it this way:
Forest: Hey Cronos, does God keep all of His promises?
Cronos: Well yes, of course.
Forest: Therefore God is a slave to His own promises, isn't He? He doesn't have the power to change His mind, does He? You don't think God is all powerful, etc. and blah blah blah..
You would correctly point out that God keeping His promises is no restriction on His power. It is not what God CAN do, it is what He WILL do. It is the same with His plan. God is not restricted, in power, in any way.