So you don't want your player's to treat martial violence as the answer to every problem. You proceed to let them roll a party that can't do much aside from kill things, and thrust them into an un-winnable fight in a nigh-inescapable location, with presumably no knowledge that it is in fact unwinnable due to their limited game experience, coupled with the fact that the players will be assuming fight is the answer, since they can't use magic or skills, and you havent indicated they need to flee. And you come here asking why you TPK'ed?