As xinetd is depricated since SLES 15 you should think about using systemd (but there it’s not possible to permit only specific source adresses) or using an extra user and get the agent data via ssh.
Just create a user and add check_mk_agent to sudoers. Also add the check_mk_agent command to the authorized_keys file.
BTW: the xinetd package is in the sles15 packagehub channel. You don’t have to use a community repo from opensuse in this case. 