Submodule's files are not checked out

Question

I'm trying to add an external repo as a submodule of my repo, so I followed these instructions, doing:

git submodule add git:... vendor
git submodule init
git submodule update

then I tired:

git submodule init vendor
git submodule update vendor

The submodule that I'm adding has submodules, and the submodule's submodules appear to to be checkedout (ie: I see the files on my hard drive) but the files for the submodule itself are not checkout out.

Any idea what I'm doing wrong?

This is the repo that I'm trying to add as a submodule, and it's src directory is all that I see in my checkout/clone, along with the sub directories and files or src, but I don't have the readme file for example.