He didn't say that Anakin could destroy the Emperor in ROTS. Lucas said that Anakin could have beat the Emperor if he didn't get all beat up. This means that after the events of ROTS, when Anakin has grown in power, he would be stronger than the Emperor and able to defeat him. I know this because of all the official evidence indicating that Anakin is weaker than the Emperor during ROTS, which I previously outlined in my posts. The OS says the Emperor is the strongest dark side practicioner during ROTS, meaning he's stronger than Anakin. This proves that Lucas was talking about what Anakin could do in the future, not that Anakin was stronger than Palpatine in ROTS. The Emperor said Luke could "destroy" him in ESB. Obviously, Luke wasn't at the necessary skill level to beat the Emperor at that point. He was referring to what Luke could do in the future, just as Lucas was referring to what Anakin could do in the future. Lucas called the Emperor the "toughest" in regards to his duel with Yoda, which means he's stronger than Anakin. McDiarmid said that his character is "much more powerful" than Anakin. Master Replicas said the Emperor and Yoda are the greatest wizards of the Force. Yoda sent Obi-Wan to fight Anakin, showing he considers Anakin the weaker Sith lord. Anakin lost to an opponent who can't even "compete" with the Emperor. The Emperor said "Darth Vader will become more powerful than either of us", meaning he isn't as strong yet. Lucas said "you have to be either Mace or Yoda to compete with the Emperor. This means Obi-Wan doesn't even have a chance against the Emperor, as Yoda said. Obi-Wan's lightsaber skills are inferior to the Emperor's. Gillard said the Emperor is a level 9 swordsman, but Obi-Wan is only a level 8. The Emperor's lightning is so strong that Obi-Wan would probably be easily electrocuted.