PAX Round-Up: Orcs Must Die: Unchained and Party Hard

"Player vs. player combat can and does happen in of Orcs Must Die: Unchained, but it is not encouraged"

This is straight up incorrect. Player vs Player combat is augmented by the presence of minions and traps, but it still makes up the majority of Orcs Must Die! Unchained gameplay, and is highly encouraged.
Each team has players dedicated to assaulting the enemies defense, the enemies defenders fight the attackers off as they try to escort minions through the traps, this is where most of the PVP action is.