User blog comment:B214/Question and Answers 2/@comment-187.209.0.247-20140225180954/@comment-16281343-20140225181654

In Volume 12, Truth Idea chose Issei's dream and that's it.

In Volume 17, Cao Cao is going to join DxD. That's why he helped them destroy the barrier, most probably erected by Ladon, by using the TL.

Those events has nothing to do with Issei being a potential user of TL.