You have to ask him what exactly he meant by it. If the zombies can't see or reach you though they attack structure and have been for a long time. Additionally they randomly go into destruction mode even if they could reach you. So what is missing there in your opinion?
Also, I don't know if I ever have played a "traditional" tower defense, but I have played a few and in all of those the enemies have not attacked any defense towers but were simply walking to their destination, the central tower (as far as I remember). If you were controlling a toon in the game yourself they would attack you though. Now in 7D2D you are essentially the central tower and the zombies already attack you. So I don't think you can argue with tradition in this case. Except if you played the original mod from warcraft and can tell me what happened there, I have no knowledge of how the mod worked in that regard.