Things like this work because of the shock value; the more intense and daring, the better the odds of throwing everyone off guard.
Security guards are trained to handle drunks and people causing fights, or just troublemakers. They aren't really equipped nor trained to handle people with automatic rifles and foot long + hunting knives.
There's actually been a few cases of similar issues in the past, it usually involves finding routine times when a money transfer is being made from an armoured truck; when its' at its' most vulnerable and easiest to just grab and run with since it'll only have a few lightly armed guards at best.
There was a string of similar ones in vegas awhile ago I saw on a documentary on top end robberies. Eventually they got caught, in the first time that High def security cam footage had been released publically by a casino. Forget the details past that.
So it's not without prescident.
It doesn't matter if they're right. If they can't proove they're right, then they're wrong. No matter how right they may be.