The kernel source is pulled in by the respective .nix to build the kernel. Eg. the following defines which commit of zen-kernel should be used as source.
https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specifi...
We are talking about a RHEL clone. Specifically the lack of dist-git pkg for the most heavily changed package that differentiates RHEL from an upstream distro. The forked Linux kernel source code.
This file does not point to that EL fork. Just the upstream with the zen patches... I guess? Not looking at it too much past that 6.6 value in the first few lines.