It's been forever since I've had their beer, but my recollection was that their problem wasn't diacetyl. The way people perceived it was super variable, with some saying it was buttered popcorn and others saying it was completely fine, when perceptions of diacetyl didn't really vary. If you think about flaws that sometimes taste like butter but sometimes don't, I think you're left with some butyric acid derivative, which makes more sense to me.
Now this could be wrong and it could have changed, but I've always thought it was interesting about their beer.