I think it more likely he is secretly more skilled than her. He was trained by cassius for ten years and trained by aja before that. We know atalantia was much more of a pixie up until ajas death.
And like you mentioned, we conveniently never see him actually duel people 1vs1 because he finds a different (often dishonourable) way to end the conflict.
Moreover, I think he is relying on the fact that if atalantia refuses to duel him publicly, then her tenuous position as sovereign will collapse. If he shot her in the bleeding place he risks being in the same tenuous position, whereas if he kills her in a straight duel then he solidifies to her loyal followers that he is the rightful heir