Was it in Satan's power to give Jesus all the kingdoms of the world?
Yes, the context states it was absolutely in Satan's power to give those kingdoms, because since Adam fell, Satan is the god of this world. Satan's audacious offer to give Jesus all the kingdoms if He worshiped him rested on that reality.
Les teaches that the devil took Jesus up into an exceeding high mountain and showed Him all the kingdoms of the world. Jerusalem sat near the center of the then-known world, with the Roman Empire extending west to Great Britain and east to India, involving North Africa and Egypt. From that vantage point, spiritual beings and Christ could envision the great empires of history and even the future. Satan then said that if Jesus would fall down and worship him, He could have all of it. Les asks whether it was in Satan's power to give those kingdoms, and answers, "Yes, absolutely," because since Adam fell, Satan is the god of this world.