I never said it was trivial. And when I said it works or doesn't work, the implication was that transforms more naturally map to requirements, not that the requirements are always understood.
I guess I could have written a more complicated article full of all of these caveats, but I'm happy that testing and developing functional code is much different than imperative. Things like parsers and ASTs are more complex concepts, sure, but they are common concepts that leave little room for doubt.
I think data structures become much more important in FP for some of the reasons you've outlined. Hence my comment that those OOP skills and Codd comes in handy. This is also why used the SQL example.
But thanks for clarifying yourself. At least now I know that I disagree with you and you misunderstood the article, instead of just wondering wtf you were talking about. I'll make a note on the article to help clear up this confusion.