Declarative has a few definitions with respect to computer languages, and the Nix community seems to be using one and ignoring others.

When I think of declarative, I think of a language that has less power than a Turing-complete language. Think Make without bash or any shell. Or HTML without CSS.

In other words, the Nix community seems to use "declarative" as meaning "without side effects" where some programmers, like myself, hear "declarative" and think of something like HTML without CSS or JSON.