Ah, one of the recursive WTFs in the IPv6 design.

Technically, there's no broadcast in IPv6, so the host is supposed to join the local multicast group and do the neighbor discovery flow to find the "on link" address. And it's not guaranteed that the network is "symmetric".

Technically, this is also true for IPv4. You can have a proxy-ARP host impersonating the sender, but since it had never been fully specced, nobody cares about this scenario.