It’s not that they’re different words. It’s that the word “law” has multiple distinct meanings. One (the more common meaning) is a synonym for “statute.” That’s not what’s discussed here.

Your second paragraph is actually exemplifying the exact type of misunderstanding of the situation that I was alluding to. You really don’t need secret legislation, or even secret statutes, or even secret rules to produce the “secret law” referred to in the article. All you need is secret decisions.