I think that the author is a bit confused about email validation.
When I was doing this, via email, if you wanted a certificate for sub.subdomain.example.com - the list of email addresses were in order something like hostmaster@sub.subdomain.example.com and hostmaster@example.com - you clicked the radio option that best suited you and you were good to go. You don't need email addresses for every subdomain.
I think the reason they couldn't do it is that they want a certificate for *.sub.example.com. Wildcards tend to trip up certificate provisioning in annoying ways.
When I was doing this, via email, if you wanted a certificate for sub.subdomain.example.com - the list of email addresses were in order something like hostmaster@sub.subdomain.example.com and hostmaster@example.com - you clicked the radio option that best suited you and you were good to go. You don't need email addresses for every subdomain.