You forked the Drupal repo? Does it already have sub modules added in .gitmodules? If so you only needed to clone their branch and perform
git submodule init
git submodule update
You don’t need to re-add their own sub modules to the repo.
Now if you want to add additional submodules you have to perform git submodule init; git submodule update every time you clone the repo. It will not automatically get the submodules.