An NPC does not abide by player standards. This mean a NPC Death Knight is not a Death Knight player class. It is an NPC Death Knight. This means they can do things normal players can't, and it wouldn't affect our own image of the Death Knight player class. NPC Engineers are not players. They are not characters that took the Engineering Profession, because that would be using
game terms to define them. NPC Engineers and Tinkers are simply NPCs; wholly unrelated to Player class discussion.
Engineering can not be defined any more than the term 'Warrior' can, since the literal meaning would apply to every player who fights in a war.