how does Yggdrasil work if ipv6 is dead?

Yggdrasil uses exclusively IPv6 for routing on its own overlay network, but not on the Internet. See https://yggdrasil-network.github.io/faq.html#does-yggdrasil-...

It forms its own IPv6 network separate from the internet. It can peer over any IPv4 or IPv6 network, including the internet, LAN, or PTP.