User blog comment:B214/Question and Answers 2/@comment-187.209.0.247-20140225185340/@comment-24501441-20140225190342

There really isn't any way for Issei to gain True Longinus. Cao Cao is still alive meaning he is still in possession of it. Truth Idea only selected Issei's dream over Cao Cao's ambition during that fight, and as Cao Cao later said it was because he had lost his way. It didn't turn against Cao Cao, as demonstrated when Cao Cao was still able to use it to assist in his escape. As said before, Cao Cao has joined DxD, and that would explain why TL came to destroy the barrier. His demonic power has absolutely nothing to do with this issue.