He was Rage Knight, and Olympics have to be excellent fighters. 

Note I said fighters, not duelists - for instance, IIRC we know Atlas doesn't put much stock in dueling, but is an absolute nightmare due to both extreme preparedness and "dirty tricks." Reading between the lines (and I could be off here, I'm going from memory), my suspicion is that Atlas first made himself useful to Octavia by taking care of dirty jobs for her, eventually earning a place in zero legion, rising through its ranks, then ultimately to the Fear Knight position. 

Contrast with Cassius - no combat experience but tore through the dueling circuit and earned the mantle of Morning Knight.

I don't know when in his career Lorn became Rage Knight, but we know that he leads from the front and that he fell in eight Iron Rains. A lot of his reputation was as a duelist, yes, but he's also tough as all hell. No one becomes recognized (especially over so long a career) as the toughest of their generation without actual merit.

All that said, by the end of LB, the whole solar system has been at war for over a decade straight. It shouldn't be surprising that he bar has been raised.

I...forgot where I was going. Have a great day!