Hi all. I'm Jesper who wrote the blog referenced here. I just wanted to clarify some things about the cloud domain join feature.
Azure AD logins - the UPN - is required to have a public domain to determine where the authentication is happening. By default when you create an Azure AD (AAD) the UPN will be *.onmicrosoft.com where the wildcard is the name of your AAD. If you want to use a custom domain, cloudpuzzles.net as in the screenshot above, you have to add that domain to AAD and verify it. That way AAD can forward login requests for *@cloudpuzzles.net to AAD, and if I want to, I can also have it forward these requests to my ADFS server. If you're syncing users from your own AD you should add the UPN.
Also, a computer cannot be both cloud domain joined and joined to a normal AD, so yes, a workgroup for the computer.
And yes, there is no doubt still a lot of work going on behind the scenes.