That agent (Python, single-file https://github.com/userify/shim) sticks with decentralized regular keys and only centralizes the control plane, which seems to be more reliable in case your auth server goes offline - you can still login to your servers (obviously no new users or updates to existing keys). It just automates user and sudo configuration using things like adduser and /etc/sudoers.d. (It also actively kills user sessions and removes the user account when they're deleted, which is great for when you're walking someone out in case they have cron-jobs or a long-running tmux session with a revenge script.)
This project looks powerful but with a lot of heavy dependencies, which seem like an increased surface area (like Userify's Active Directory integration, but at least that's optional)