That vision is ambiguous no, since it says “as much as the devil asks for” which could mean only a fraction of his power (since the devil may not ask for full power since he knows God would not agree to such terms). Plus if the devil is fully unleashed, then something which has never happened since the dawn of time would’ve occurred (namely that the devils are fully unleashed) in which case, God would have no concern for mankind (since man is weak,stupid, and utterly defenseless in compared with even the weakest demons) which is false. If the devils were given full power, then why does the world still exist? One would think that they would destroy it? Or why is there still hope for mankind, since one would think that the devils would’ve already overturned the Church?
The vision was not ambiguous. It was auditory; the Pope experienced it as overhearing a conversation between Satan and Jesus. It was more or less the following:
Satan: I can destroy your Church!
Jesus: You can? Then go ahead and do so.
Satan: I need more power and more time.
Jesus: How much power? How much time?
Satan: I need absolute power over those who will give themselves to my service, and seventy-five to a hundred years.
Jesus: You shall have all the power you ask for. Which century would you like?
Satan: The twentieth.
Jesus: Done. Do your worst!
And the rest, as they say, is history.
Satan is a finite being. His power and intellect are vast, but not infinite. He knows more about his own nature than you do. Your conjecture that God limited the power He granted him to destroy His Church is not based on a real understanding of angelic power. Satan did everything in his power to destroy God’s Church. He failed, not because God bested him in a battle of strength, but because God is Love, and hate is inferior to Love.
God already demonstrated His Omnipotence in a battle of strength, when He led the Israelites out of Egypt. Since that was a battle of strength, God’s Victory was real but not permanent, because Strength is not the same thing as Love. The Final Battle is not a battle of strength, but a battle of the Spirit of Love against the spirit of iniquity. It is a spiritual battle, not a physical one. God did not throw chariots into the Sea, when He defeated Satan definitively at the close of the twentieth century. Rather, He increased His graces of Love and of conversion from sin through His Immaculate Mother’s Rosary. And that increase, is still continuing to this day, and will continue, until sin is no more. It is through the Rosary that God demonstrates His Omnipotence of His Love over Satan and his legions of evil.