The issue was with downloading macOS Catalina. Had you the same issue as us, your method would not have worked because you wouldn't have been able to download Catalina.
That might not be necessarily accurate. I have always downloaded the full installation file, instead of choosing to do an update, and then stopped the subsequent installation process. If one chooses to do an update, after the download, it immediately starts the update process.
But I'll bet other folks have been successful doing an update. So not sure why this issue came up. I just know that by downloading the full installation, stopping the subsequent "automatic" installation, and then doing that installation myself (and thus having much more control), I've never had any issues.