>> C programs aren't functional because C isn't functional (no inherent concept of closures)

You don't need closures to do functional programming.

I was recently tinkering in some C++ code and thought "this part should be able to run in parallel." It was simply doing the same operation of a list of objects. It's a complex operation involving some temporary data structures. I had to dig through several layers of functions, but I kept finding that all data was created locally. Functions that returned complex result are passed a class to fill out. It was then that I realized it was written in a functional style - or at least my limited understanding of what that means. The top level function is now run in parallel, which required no changes beneath that level.

Now I prefer that style but like TFA here, I don't think 100 percent functional is a good idea.