Comment by Dagonfly

You were asking about reachable, not routable. Of course, an address behind a firewall might be adressable and theoretically routable, but it certainly isn't reachable.

The default config of a SoHo router has both: (a) NAT44 separating public vs. private IPv4, (b) and also a firewall that blocks any unknown ingress traffic on IPv4 and IPv6.

From a security standpoint, it doesn't matter if the IP get translated inbetween. Only the ingress firewall counts: If the firewall is in-place both IPv4 and IPv6 traffic will be blocked. If you open up the firewall (via hole punching or port forwarding on IPv4, or via IP allows for IPv6), I can send packets to your public IPv4 + port and they will arrive at you private server, or I send them to your servers GUA IPv6 and it arrives. There is no security difference.