Designated servers offer no technically advantageous feature set for P2P. They simply keep the P2P traffic separate from other traffic so that non P2P server latencies remain lower and more responsive.
It gives Nord something to advertise as a special service, when it really is only technically advantageous to them that people use those servers for P2P, and not the other servers. If they really wanted offer P2P relevant services, they would have port forwarding available. Basically. by offering P2P servers, they are saying that on those designated servers they do not block P2P traffic. That is the difference.