I definitely agree there! I’m not totally sure why it exists. Perhaps Pulp 2 is trying to shield clients from “bad” errata metadata. I did find an old Pulp issue that might be related Issue #1548: published errata contain packages not in repo - RPM Support - Pulp
Pulp 3 for Smart Proxies probably won’t come until Katello 4.0, which is when Pulp 3 will fully replace Pulp 2.
I think this is a good idea for a workaround. I’m not sure if Katello or Pulp 3 will mind the duplicate errata, but it’s definitely something to try. I’d be curious to hear what happens if you do.
One thing to look out for is a community solution to importing CentOS errata for Pulp 3. I know people made it work for Pulp 2, and I’m guessing someone is working on it for Pulp 3. Katello 3.16rc5 and pulp-admin wrt CentOS Errata - #14 by Michael