Right. Nix bootstraps from a fixed collection of seed binaries, and everything in any given commit of nixpkgs is derived from the seed binaries and versions of software that exist in that commit of nixpkgs. It doesn’t solve the “trusting trust” problem with those seed binaries, but it does solve the “arbitrarily long chains” problem.

(The work at https://www.gnu.org/software/mes/ aims to minimize the needed binary seed to a human-auditable size, and should be applicable to Nix and Guix.)