Yes but we're not comparing distribution-official APT repositories with unsigned downloads, we're comparing potentially third party APT repositories with HTTPS

Yes but we're not comparing distribution-official APT repositories with unsigned downloads, we're comparing potentially third party APT repositories with HTTPS downloads. When users add a third party repo they often don't check who the author is, who the key belongs to or whether they even obtained a legit key. In the HTTPS case, users don't check the HTTPS certificate. The end security is about the same.