As much as Eldrian wanted to ask what Ziraili meant, he was in no condition to do so. While he had suffered no injuries in the fight—physically—that did not mean he wasn't tired and drained.
By using Tranquility as he had, he had expended a vast amount of energy. Yes, this energy was the XP he had in excess. It did not touch on his soul world: The manifested world with form—excess XP is the void extended past the 'physical' world within Eldrian's soul.
Having the excess lifeforce in the from of XP helped. It was like fat Eldrian could burn before needing to burn his own muscles. However, using it still put a drain on him.
While he might have the energy to burn, that didn't mean he could use it without repercussions. This tired state of his was the cost of overly drawing on a power he had yet to learn full control of.
