Are those really the same package definitions? The guix one contains build instructions and the nix one is for binary packages.

No they aren't. I was pointing to the fact that in Guile you don't need shell scripting where you can use functions like 'mkdir', 'symlink', 'copy-file', 'file-system-tree', etc..., so you can use scheme with all the support Emacs, Geiser and guix.el give you.