Umm so I looked again and tried a few things, and it seems that maybe I phrased the question wrong.
When I open the dashboard and click the Registrar section, I don’t get a prompt for the domain name to transfer. Instead, it asks “Select domains to transfer”. Indicating that I do have to go through the whole exercise of ‘Add new site’. And it will not let me proceed until I alter the authoritative nameservers at the old registrar - before I even initiate the process. And there may be further steps required.
So yeah, even though the service is technically separate, one still has to go through the motions of doing a full site setup (and probably using ‘skip’ for most of the steps), and the ‘site’ will show up in the dashboard with the UI for CF site management. There is no standalone lightweight dashboard for just the registrar stuff.