Yep, that is intentional. While this comes across as a positive attribute towards people who are already functional programmers, I don’t think it’s a good pitch to a more general public - arguable it’s a bit misleading (given the lumps of impurity and imperative configuration in current-day NixOS), and it’s a term that just doesn’t mean anything to a lot of people, other than “those academic people”.
I think there’s a lot more value in describing the consequences of Nix’ design for end users, and leaving the ‘specialized’ term for it up in the air. This sidesteps preconceived notions, and attracts interest from people who aren’t familiar with the term or paradigm at all, but who might nevertheless appreciate the qualities afforded by it.
I’d apply this even to a single-line slogan; rather than “the purely functional OS”, for example, “the reliable and customizable OS” (or something along those lines) might be a better pitch - because it immediately communicates to people why they should care, regardless of their existing background.