Yeah, I reached out to their support a while ago to ask about it and they made me aware of the CardDav/CalDav instance at cdav.migadu.com. But yes, my understanding is the same as yours in that it is an unadvertised beta feature.

I can't remember the ports but if you point a CardDav/CalDav client at cdav.migadu.com they pick up the settings automatically. I've done this on iOS, Android, MacOS and Linux and they all seem to work perfectly for me with Migadu.