The devil is a LIAR. Period. Fr. Amorth should know that already. If he's saying that one who prayed side by side with satanists and animists, and other PAGAN SATANISTS can be "effective" against the devil, I would actually laugh at that.
The devil is THE FATHER OF LIES, and anything to further his cause in taking souls, he will do. People idolize JPII. The WORLD does. Pretty sure Fr. Amorth is being tricked here.