No, it isn't. Many middleboxes (including OpenWrt by default) drop unsolicited inbound TCP connections even on IPv6, and therefore the same hole-punching algorithm is needed. The hole being punched is in the stateful firewall's connection tracker, not in the NAT. Basically, both parties need to convince their router that it is an outgoing connection initiated by them, not a prohibited-by-policy incoming connection.
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No, it isn't. Many middleboxes (including OpenWrt by default) drop unsolicited inbound TCP connections even on IPv6, and therefore the same hole-punching algorithm is needed. The hole being punched is in the stateful firewall's connection tracker, not in the NAT. Basically, both parties need to convince their router that it is an outgoing connection initiated by them, not a prohibited-by-policy incoming connection.