Is the thickest string muted when playing A#5 and C#5 power chords? It looks like Justin isn't muting it with his index and I really can tell from the sound.
I know this is extremely late, but yes you are supposed to mute the low E string. Use your ears my man. I know when I was learning this song, that low E sounded like dog crap when I let it ring out. A simple solution for an awesome, "easy" song. But just an example of how using your ears and listening to the actual recording, you can determine what you should play.