For DHCP registration, dhclient passes the hostname via a parameter in /etc/dhclient.conf.  This is why SUSE has used dhcpcd in the past since you can manipulate more dhcp options without resorting to hard values inside of a file (makes dynamic changes easier).  Since the send-hostname is commented out and NetworkManager fails to edit the file to insert a value, any DHCP connection made with NetworkManager will fail to dynamically register a host name (via the dhcp server for example) with DNS.