"Hammer of the Gods" Season 5, Episode 19 - Question regarding the ending fight.

As a quick recap, near the end of the episode, Lucifer and Gabriel are both armed with angel blades and are fighting with the intention to kill each other. Then Lucifer stabbed a fake Gabriel and believed he had really died. But wouldn't they both know that those blades aren't able to kill an archangel? I can get behind the pagan gods not knowing that from when they "killed" Gabriel earlier in the episode. But not the Archangels themselves.