Leaked screenshots of internal documentation and messages reveled by an unknown source confirm a myth that the Kubernetes team prefers simple Bash scripts for own deployment tasks.
It seems like in the most difficult deployment scenarios, team leaders at k8s sometimes may additionally use Makefiles. In the worst-case scenarios Ansible is used. However, no evidence of using Kubernetes itself has been found.
A leaked screenshot of the internal Jira board shows a ticket "Figure out fckn k8s deploy for staging-2" that is "in progress" since January 2018.