I am surprised the seat wasn't taken by a non-rev

Not all international business class seats are offered buy-up at the gate on AA or UA either.

How an airline sell/allocate premium cabin seats, especially on long-haul international flights, is really upto people working at revenue management (RM). Many US based airlines offer them as perks to employees + their friends/family as non-rev. Many European/Asians allow premium cabin seating to only certain non-revs (management and pilots). Many of these same airlines don't even offer mileage upgrades last minute, whereas most US airlines will sell premium cabin seats to those upgrading with mileage+copay.

At the end of the day, it is up to the RM to decide and they usually employ a lot of statistics to maximize profit for the airline.