Neither knock nor knockd depends on iptables. Still net-firewall/iptables is listed as an RDEPEND in the ebuild for net-misc/knock-0.5. Iptables is only needed when you run knockd with the example configuration.
I see your point, You can use knockd to trigger other operations like /sbin/halt on a given sequence instead of running iptables. But iptables certainly is the default usage. IUSE minimal?
+*knock-0.5-r1 (31 Aug 2010) + + 31 Aug 2010; Michael Weber <xmw@gentoo.org> +knock-0.5-r1.ebuild, + metadata.xml: + Revision bump to fix bug #332897, thanks to Sebastian + <indigo176@blinkenshell.org>. Die more, EAPI-2.