I think its both....depends on which season, but generally speaking, east-based la nina events, like modoki el nino events, tend to be less pervasive forces in the atmosphere (though this one was very prominent early on)....so it makes sense that some of the disconnect from la nina climo may be a byproduct of that. But its also due to the fact that the forcing tends to make it farther to the east, closer to the dateline in east-based la nina events, like Scott was illustrating via VP image.
Anyway, just as you say that everything is caused by an event in the atmosphere, I am of the opinion that there is rarely one "silver bullet" or "smoking gun", so to speak. In fact, its usually an agglomeration of factors working in concert and vying for proxy.
The HC stuff totally makes sense to me...my aim wasn't to dispute that in any way....just want to be careful not to overattribute phenomena to it.