I think he means it as in when you and a friend are targeting the same mobs the entire time while questing for example. Which would mean you indeed do kill the same mobs, but in my case I never fight against the same mob with my friend. I only do that in certain circumstances like when we fight against a boss. But again just because you kill faster doesn't really fill the loss of half the exp in my opinion.
I'm just waiting his explanation,he may have a good argument about it.