This is true. However....

The basic premise that you are operating with mathematical functions, which allow you to reason equationally can be done in any language.

Of course when you move from C to Perl you pick up a lot of power in what you can do functionally, and when you move from Perl to Common Lisp you pick up even more, and when you move to Scala or Haskell, you pick up even more from what I have seen.

But just as you can do object-oriented programming in C (a language really not designed for it), you can do functional programming in a lot of languages not really designed for it too.