I don't see how this is specific to "exploiting CI / CD Pipelines" when he's really just exploiting someone encoding their github username AND password credentials (unorthodox af) into the url for remote.
- they use that credentials to make a commit adding malicious code to the CI pipeline
- The rouge pipeline job adds their public SSH key to the `.allowed_keys` file in the production server
As the pipeline is run automatically on push, they get ssh access to the remote server.
That is the "CI / CD Pipelines" bit. That being said, it's a bit underwhelming, because given the title I though they were going to exploit a bug in the CI/CD software itself. I don't know if I'd call that "exploiting" CI/CD software.
It sure wasn’t a good decision to use git-config to store creds for CI though! I wonder if OP found a developer’s old cached creds in the history that weren’t used anymore but happened to still be valid?