If anything, it was the other Forsaken, not Ishamael/Moridin who were duped by the title of Nae'blis. Moridin knows exactly what the DO and is willing to achieve it. Demandred doesn't. Recall from his POV in WH where he thinks about how crazy Moridin is, he thinks that some of his plans and order were nothing but sheer madness. Well, they weren't madness because the goal was the same as the DO's: to destroy all of creation. Naturally, Demandred thinks it's madness because the annihilation of everything isn't his goal, it's to rule over the whole world or at least his own private empire.

The whole title of Nae'blis was created by the DO because he wasn't happy with the infighting among the Forsaken and he was trying to bring them to heel. It's probably no coincidence that the first we hear of Nae'blis is in LoC when Moridin's soul is still being "processed" by the DO and Ishy isn't around calling the shots. And the DO is enforcing his will, not just with the empty title, but by Moridin's exclusive access to the TP and through Shaidar Haran.