Like wordpress.org has wordpress.com, they could have had this commercial domain. They could even have had a signup form on their .org website that would return “Your account has been created! Yours is on [xyz].com! But don’t worry, it will connect to other accounts from all others of the federation.”
luckily you didn't need to guess, because it's presented as the first entry, including the information that it is run by the company, when you go look at the .org page for where to sign up, just as you wanted.
But I'm sure you'll find another reason why doing exactly what you suggested isn't reasonable.
