Of course they can't. But the point is that if a client wants to interact with IPv6, they only have to upgrade themselves, and not wait for everything in between to get upgraded. And the service can serve on their IPv4 gateway and then pass the traffic back to other hosts to server IPv4 clients.
I had idea of putting the internal address for NAT in options so could be addresses directly. But too many boxes would need to be changed, and lots of unchanged boxes remove options. Plus, all the IPv4 applications wouldn't know what to do with stack of addresses. It would devolve to NAT.