I really liked Armin failing last chapter, too. He might believe in his own strength more, but he's still inexperienced as a strategist and as a leader, while Jean was the one who was thrown into a leader position back at Trost and is naturally better at it because he isn't so strongly effected by pressure/mind-oriented and contemplative over things (such as consequences). He learned to act when it's needed last arc, while Armin is just really aware what his choices and logic could lead up to. Bert's words didn't help, either.
Not sure how to phrase it better, but basically this was some great character exploration.