I figured it out.

If the List settings have the list experience set to "Classic" (which seems normal for that older type of sharepoint situation), then an SPFx extension will not load.

If the List settings changed to "new experience" then the SPFx can be loaded.

So, in the unmodified pre-existing scenario, the SPFx would not load (and there was no redirect), because it uses the classic experience. When the new application is released, part of the release process will just have to be to install the SPFx with the redirect, and then change the list settings to use the new experience - No SharePoint Designer required.