The wording is confusing. They used an Ubuntu build system to run this script, which produces the firmware file to be flashed to the ESP32. The firmware is a minimal Linux-based OS, not Ubuntu.

https://gist.github.com/jcmvbkbc/316e6da728021c8ff670a24e674...

It just runs the kernel and one CLI app, maybe two. No ui, no swap.

It is the same kind of rootfs your laptop runs, but the similarities end there.