How can leaders avoid the Jeune École's confirmation bias?

Was the Jeune École's problem that advocates assumed the enemy would do exactly what made the idea work?

Yes; the doctrine depended on favorable enemy behavior, which is why leaders need trusted subordinates willing to challenge their plans.

An attractive strategy can become self-confirming when its advocates assume that the enemy will behave exactly as it requires. The proposed safeguard is a chief of staff or second-in-command who has the confidence to question a plan. Constructive challenge differs from bullying, personal belittlement or obstructing everything for its own sake. An idea that survives a trusted subordinate's objections has received a more useful test than agreement alone.