Indeed, a host should be able to request a /64 via DHCPv6-PD and split that between millions of container networks. But you can't do that on Hetzner (or anywhere else).
Yeah that obviously only works on /56 and above because networks should be a minimum of /64. I use k3s and each host has a /64; cilium just gives each pod a /80 and the host does NDP and stuff. Works fine, no need to require dhcp6.