From what I can tell, the issue here is that "git submodule update" does not cause any error if the repo for a submodule is missing. However, once the bad .gitmodules change is merged, Jenkins will attempt to check that submodule out using
git config --get submodule.sub/module/path.url
Since this will cause an error on a missing path, the verify job needs to test this condition in order to prevent job-breaking merges.