Either she's supposed to have known given that she had a high profile marriage into the Mo family or the fact that she spent a decent amount of time inside the ancestral temple
he's saying she should've realized it sooner, when she first tried to grab the sword, and if she had let go immediately, she would've sustained no injuries or only light ones compared to what she actually has.
Who the hell says it's not Excalibur? You? Maybe it actually is very well-known or supposed to be well-known. There's no evidence either way.
That just proves my point. Maybe the male MC expected her to have been taught by someone. Anyone. She is a member of the household now. He had no idea his stepmother was being hostile to her. This supports my point.
And finally, a weapon that you cannot use is useless.