I’ve just double checked your Debian 10 repositories: looks identical to the blog post on opensource.com as well as a test system I use:
debian main:
Upstream URL: http://ftp.debian.org/debian/
Releases: buster
Components: main
Architectures: amd64
debian security:
Upstream URL: http://deb.debian.org/debian-security/
Releases: buster/updates
Components: main
Architectures: amd64
debian updates:
Upstream URL: http://ftp.debian.org/debian/
Releases: buster-updates
Components: main
Architectures: amd64
I am confident these settings work.
I suspect it’s rather got something to do with Katello and pulp/pulp_deb.
Can you please share which version you are using as well as a more percise error message?