There are API functions avilable for all types of records in ispconfig (clients, webs, ftp accounts, email domains, mailboxes, dns records etc.) So the howto you pointed to is just a example for ftp users which touches only a small fraction of the evailable API function. The full API can be found in the file /usr/local/ispconfig/ispconfig/interface/lib/classes/remoting.inc.php
btw, are there any problem to create mailbox domains without any client id associated ?
No. But its recommended to create at least one client. Without a client, you cant use the API.