Aymeric's armor is only "heavy armor" as far as stats are concerned on the player wearable equivalent. Assuming a tank is wearing it. Also, he wasn't fighting the peasant. He was basically saying "Hello!" and got shanked. Surprise!

As far as the assumed material goes, it provides very little protection. And knives are actually really effective at stabbing through leather/cloth. You'd be good to call BS if he got stabbed in the shoulder though.