Nope you are not an idiot at all! It happens to me all the time with that deadly mk1 back-swing. It's killed me in Bronze, it's killed me in Titanium, it's killed me a whole heck of a lot and it still kills me today. I drop my guard around those guys and will run behind them, half ignoring them completely. Well woe is me when I get killed for the Nth time that round.
It's all about timing and the fact that those swords are live as soon as they start swinging! :-)