In addition to the missing bib-config.toml file mentioned in the Getting Started section of the article, I also see that the steps have changed slightly. It looks like the image builder no longer pulls images for you when you execute the first step:
bootc-image-builder no longer pulls images, make sure to pull it before running bootc-image-builder:
sudo podman pull quay.io/hummingbird-community/bootc-os:latest
At least the error message explains how to correct it but, it may be good to update the “Getting Started” section.