Same thing happened when we used the blade of light on Lahabrea when he was possessing Thancred, it pretty much forced him out of the body like Emily Rose being exorcised. The white Auracite pretty much locks the ascian in place so the blade can actually function as a blade.
Well it was mentioned out of the mouth herself that she could split things like that (Word of Mother Quest). Also Emet Selch also pretty much confirmed that Hydaelyn had the capability and since Hydaelyn said it (kinda) first, I do believe Emet on it.