He is probably referring to the thing you see when you look into a .drv file, aka the immediate result of evaluating a derivation in nix. That language/datastructure is what nix-build evaluates to actually build things, otherwise the nix language itself is purely functional and cannot really do anything on it's own.

I guess in guile/scheme the equivalent data structure happens to be an s-expression. Edit: except the paper actually describes that it's a g-expression.