But, if I understand correctly, this is not needed. One simply needs to allow root login (per the quote above) from the slave (second or third or 4th ...) server to the master (the one from which all servers will be administered through that single control panel.)
Yes, thats correct. ISPConfig is doing the replication automatically. You do not have to setup mysql replication. If you setup mysql replication ispconfig would not work.
This thread is about a different topic. Its about replication in a high availability setup were all nodes host the same sites while ispconfig is doing a multi server setup were every node hosts diferent sites.