Not an apt user but my quick search suggests this just downloads the source of the package? The guix shell --development command in this article is for getting all the dependencies needed to build the package (but not the source itself, though you can do that through other guix commands I believe). In other words, with guix shell you can now run make or whatever you need to build the package in question, without needing to fetch the development tools, other libraries, set env variables, etc.

Edit: for the source of a package guix build --source thepackage will return the path to the source (as stored in the store). This includes any patches or transformations (e.g. you can pass patches to be included or a git/branch/commit location to pull the source from instead of what is defined in the package definition)