Pedantically, you can't just redefine a term to suit your need calling something "pure" or "unpure".
No, I think they are using those terms correctly. In FP, “pure” means absolutely no side-effects. So if you add an FP-style library to an existing imperative language, that’s “impure”.
That's not the context in which the word was used, but it's a fair point. I have worked professionally with Haskell, and while I am not good at it (my forte is Erlang which is what I would consider "close to functional"), other developers I have worked with were very good, and watching them code in non functional languages and listen to how they thought about solving problems is were I developed the idea that adding a few features doesn't make a language functional.
