I believe that CocoaPods is created exactly to avoid GIT submodules usage.
Here is cool explanation why: https://roadfiresoftware.com/2013/05/a-case-for-switching-to-cocoapods-from-git-submodules/
Let’s say your project depends on AFNetworking. You add it as a git
submodule, and then, someday, due to an Act of God (or just because
the author feels like it), AFNetworking disappears from its home on
GitHub. It’s all gone, vanished into thin air. Now when someone new
clones your project repo, they run git submodule update –init …and it
fails. So now they can’t build the project. And you’d better be
careful, too, cause if you lose the AFNetworking source code, you
can’t build either…So now we use CocoaPods, and we push the source for each dependency to
our central repo. Now when someone git clones a repo, they get all the
code they need, and they can build the project just fine. They don’t
even need CocoaPods to build the app – it just works.
So, let CocoaPods do its job.
CocoaPods manages dependencies for your Xcode projects. (c) https://github.com/CocoaPods/CocoaPods