The reason is very simple: the "Divine Soul Sword Technique" itself is a very mysterious secret technique. If it could be easily understood, it wouldn't be worthy of being one of the secret techniques.
Most importantly, Elder Xuan wants to see to what extent Ye Fan can understand the Divine Soul Sword Technique.
As the Spirit Soldier of the Emperor Weapon, Elder Xuan has a very thorough understanding of Divine Souls. Although the Divine Soul Sword Technique is considered a secret technique, in his eyes, it is merely so.
However, this secret technique is perfectly suitable for Ye Fan to cultivate at this moment—Ye Fan has reached his limit in cultivating "Flying Sword Skill" to 'Shattering Void,' and it's impossible for him to make significant progress in the short term.
The "Divine Soul Sword Technique" happens to fill this gap, and since this secret technique was obtained in the Xuan Realm, it won't raise suspicion.
