Using a WWW subdomain with CNAMEs accomplishes effectively the same thing as using ALIAS on an apex domain name, and doesn't rely on anything out-of-spec or proprietary, making it easier to serve redundantly. (Did you ever wonder why google.com and facebook.com redirect to www?)
(Or is there more to ALIAS than that, which wasn't on the page in GP? Happy to be corrected if so)
(Or is there more to ALIAS than that, which wasn't on the page in GP? Happy to be corrected if so)