I mean, the tieflings in the grove are all villagers, they spent their lives working the land, maybe running shops, etc. Just because a guy is big and muscular, that doesn't mean he's going to use it to fight. Sure they were in hell, but it's not like they were there for years and years. Based on what the game conveys, they were only in Avernus for maybe a couple of weeks, tops. That's not enough to turn most people into hardened fighters. I think that might be part of the point of their storyline. That they may look frightening and intimidating, but they're just people, and those looks haveled to them being discriminated against. They just want to be able to live their lives, and the fact that they aren't able to is unfair.
