User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170414184220/@comment-10330822-20170417025856

Did you not get my reference to the ultimate spear and ultimate shield, that's what I meant. Issei's attack is pretty much the ultimate spear, while Diehauser's is pretty much the ultimate shield. The question becomes, which is stronger? If one faulters, than the other isn't the ultimate objects. Its a natural paradox. In most cases, they relent and both break, the shield succeeded in blocking the spear, but the spear succeeds in destroying the shield.

So, I agree, Diehauser and Issei's abilities shouldn't work on each other. But Diehauser isn't limited on using the ability just on Penetrate, he can use it on Issei's other moves too, and even if Issei's uses Penetrate on Worthlessness, Diehauser's demonic power would block the attack as it did during their battle.

In short, you are right, Issei has the power to attack Diehauser, but he doesn't have the strength.