I always tell my favorite bartenders for over a decade at our hangout Tiki bar to tell me if he feels I am needing a cab for my 3 mile ride back home. I just said call me a cab because if you think I am not safe to drive why would I question it because I was the guy drinking so how could I tell if I reached a certain level. He would know better than I even though I try to be careful to keep my 60+ years of no DUI's intact.
I don't know why this captain felt he was unfit to fly but on some connections requiring hours of waiting at the bar is usually where we spend our time and we try to pace our drinking but it is pretty boring to just sit at the departure gate for hours. No one has ever questioned us in decades of flying if we had been drinking.
One of our checkairmen on our previous airline was so drunk they needed a wheelchair to get him to the flight and he made it. Not saying it was good judgement but he got on as a passenger.