I guess your friend hasn't been gaming for too long. Getting together with friends to play singleplayer games used to be rather common. Even after we started getting more multiplayer games like Mario Kart, it wasn't uncommon for friends to swap controllers with singleplayer games, which certainly goes beyond simply talking with them. Party chat just offers a way to always talk with your friends rather than having to get together at someone's house like we used to.
So, in your short, your friend is very, very wrong.
Edit: With that said, there's nothing wrong if he doesn't prefer it because he doesn't want someone interrupting an important story segment, but saying "it isn't exclusively solo" is just not a very good point.