The actual source is this: https://community.ui.com/releases/UniFi-OS-Server-4-2-23/21d... but only accessible if you opt-in to the Unifi Early Access program. We are talking beta software / first release here, so any criticism needs to be looked at through that lens.
Also there is the official announcement now: https://blog.ui.com/article/introducing-unifi-os-server