I didn't get much of a diacetyl note when I tasted it a few months back.
On your next trip, you might want to taste the crust separate from the toppings. That might help narrow it down.
In certain circles, I've seen the concept of 'wok hei' incorporate more than just the intense heat of the burner. Some people ascribe something intangible, almost mystical to a heavily used wok. I only subscribe to the high heat definition. When it comes to coal ovens, though, there's definitely an intangible involved. Perhaps it's the sootiness of the poorly cleaned decks. Maybe some component coming off the burning coal is making it's way into the crust, I'm not sure. But the buttery note might be coming from the oven.
My next theory is olive oil. All non motz pies get olive oil, so perhaps they're using an extra buttery tasting brand of olive oil.
My last theory isn't so much as a theory but an observation about Romano. Pizza obsessives tend to go to high end markets (like Trader Joes) and buy top shelf romano. Non obsessives tend to gravitate towards Kraft. Pizzerias on the other hand, tend to use neither. There's a middle ground that isn't expensive, but doesn't taste like sawdust. This middle ground romano is way less complex/sheep-y than the top shelf stuff.