@efa2e20e they gave elsbeth a physical sword because she is an awesome stunt woman and could handle it safely. Her father was a grand champion of multiple martial arts so she knows her shit inside and out. The magic flame part is written in so it could stand up to a lightsaber which can cut through anything but beskar steel. This is all written into the scene to make her happy as she gets to really show off her martial arts ability on film before they're done with that character.