Was it actually in Satan's power to give Jesus all the kingdoms of the world?
Yes. Since Adam fell, Satan became the god of this world, so the kingdoms were in his power to give. The temptation was real, not empty, because Satan had authority over the world's kingdoms and could offer them if Jesus worshiped him.
Les explains that after showing Jesus all the kingdoms of the world from an exceedingly high mountain, Satan had the audacity to say that if Jesus would fall down and worship him, He could have all of it. Les then raises the question of whether it was in Satan's power to give those kingdoms, and answers, "Yes, absolutely." Many do not realize this, because since Adam fell, Satan is the god of this world. The context notes Jerusalem sat near the center of the then-known world, with the Roman Empire reaching west to Great Britain, east to India, and involving North Africa and Egypt; from that vantage, past and even future empires could be envisioned.