One of the major buildings in downtown Pittsburgh was made of something like this. It was born rusted but wouldn't rust further. Particles of rust flaked off and got in the eyes of pedestrians, so there were or are complaints.
As I understand it, protection of ferrous metals, by methods such as galvanizing, has the zinc or similar metals act as sacrifical metals which as they oxidize or degrade they stay in place but yet protect the underlying base metal. CorTen was developed for bridge structural and structual in corrosive envirnments where the base metal itself needed to form a rust-like sacrifical surface that was very thick and self-healing.
The instance cited by the article sounds more like what aluminum does -- its base metal forms its own sacrifical surface of modest thickness.
Having no traning in metalurgy, this is just my "comic book" version of an explanation of the process. (kind of like the comic book versions of calssic lit that used to be put out in the 60s)