I don't quite agree. Languages push you in different directions. Haskell pushes you toward FP; you can fight the language and "do" (pun intended) imperative programming, but it's going to be painful.

Can you do FP in any language? Maybe not COBOL, but you could do it in C with function pointers. But it's still easier to do it in Haskell than in C, and it's still easier to do imperative programming in C than in Haskell.

So you have a point. But languages have different characteristics, too, and those differences let us classify languages, if not as "FP" and "non-FP", at least as "more FP" and "less FP".