Updating a README is pretty low priority.
With the Zabbly repo enabled, you can run apt install incus-ui-canonical to get the web UI.
Debian is a ādo-ocracyā ā if no one is sufficiently motivated to package requested software then it doesnāt get done. I personally donāt use the web UI, so while I have a direct interest in maintaining the Incus packages in Debian, I donāt have the interest/best background for packaging a web application.
You might also consider Introducing Hye Ararat: A Web UI native to Incus , which looks like itās under pretty active development.