NAT is not much different to a firewall though… just because the address space is publicly routable does not mean that the router has to provide a route to it, or a consistent route

NAT works by assigning a public port for the outgoing stream different to the internal port, and it does that by inspecting packets as they go over the wire: a private machine initiates a connection, assign an arbitrary free port, and sends that packet off to the router, who then reassigns a new port, and when packets come in on that port it looks up the IP and remapped port and substitutes them

that same process can easily be true in IPv6 but you don’t need to do any remapping: the private machine initiates a connection, and the router simply marks that IP and port combination as “routable” rather than having to do mappings as well