The rumour was obviously coming from that backstabbing, villainous tactician, so it could be 100% lies for all we know. In any case the damage it caused before the barrier stopped it didn't give an impression of a most devastating attack. I'd say it was at least 80% lies. It wouldn't have stopped an Emperor (even if we ignore the fact no Emperor would have had patience to sit still until the dude was done charging his ridiculous power attack).