ssh machine credentials are passed to the ssh key agent inside of the pod/container. Then when playbook runs, the ssh agent uses that key to connect to your hosts. So in this case, ansible_ssh_private_key_file wouldn't be set.
For key rotation, you could callback to the awx server (with awx.awx.credential module, or api call) at the end of the playbook run and update that machine credential with a new ssh key.
AWX Team