It doesn't sound like it was the default, just a "password1" situation.
The article explains that it was a common combination for people to use on this type of safe - "Typical combination lock, three times clockwise - 20 - two times counterclockwise - 40 - once clockwise - 60, tried the handle and it went".
"admin", "admin"? Nope.
"admin", "passwd"? Yep!
We of course wouldn't have heard about it if it wasn't succesful. The real odds are "how many safes are there like this that people are trying all over the world, and what's the chance that someone at ONE of them would succeed." :)
When you turn in a container with a classic combination lock in the DoD world, you set the combination to 10-20-30-40. The next person issued the container opens it and sets it to a new combination.
With physical goods, the key is often sold with a default bitting. (Which is the data embodied in the key equivalent to the password.) That's because just ordering the default is often cheaper, and the contractor will often just opt for the cheapest option. It wouldn't surprise me if this also happened with combination locks.