Last time I checked CentOS didn’t publish erratas in their repos, that’s why you don’t see them. There are ways[1] how to get them from other source and “sideload” them into katello. Quick search found some tools to do that[2,3].
[1] - https://www.lisenet.com/2018/katello-import-centos-errata-into-pulp/
[2] - https://github.com/rdrgmnzs/pulp_centos_errata_import
[3] - https://github.com/nicolas-r/katello-centos-errata-import