https://cr.yp.to/djbdns/ipv6mess.html
still as relevant as the day it was written
replies(3):
That’s like saying HTTP can talk to FTP servers as long as there is an HTTP to FTP proxy.
The only thing that makes them seem compatible is there is a well formed address space in v6 that clients send v4 requests to. But it’s still v6 and a 64 proxy needs to have an actual IPv4 address to translate the source to before sending it via v4 to the actual destination.
The usual "they should have designed it to be compatible" nonsense usually comes from the crowd with zero suggestions of how to have a 32-bit addressed device send to packets to something with an address outside its universe.
Point is that djb was as wrong then as they are now.