Let me summarize my understanding of what he's saying, because I don't quite see why/how you disagree. I think you (or I) might be misunderstanding his claim.
Imagine this topology: C (client, IPv4-only) <=> R (intermediate router) <=> S (server)
My understanding of djb is he's saying that IPv6 could have been designed such that S could still serve C via only simple software updates -- this means, crucially, without the need for S to separately obtain a public IPv6 address through R, because its IPv4 address would be automatically valid for IPv6.
How can this work? Well, there are two scenarios:
1. If R is IPv4-only, then S could figure that out during some startup/negotiation process, and send only IPv4 packets to R. R only lets IPv4 clients connect to S anyway, so any response (even from IPv6 applications) on S must be going back to an IPv4 address. So the kernel can transparently translate those IPv6 addresses into IPv4 before passing them along to R, and vice-versa.
2. If R supports IPv6, then R can do the same thing S would've done in the previous^ scenario. (In fact, I think S could become IPv6-communication-only in that case, reserving IPv4 for just address leasing? I'm not sure, but in any case, I don't think that matters here.)
Notice that all of this is almost completely stateless. (I think the only state S needs to track here is 1 bit, indicating whether R supports IPv6 or not.) So, S and R can be independently and (importantly, rather trivially) upgraded to support the IPv6 protocol, without losing the ability to talk to any clients within the IPv4 address range.
This is easy and requires no explicit leasing of IPv6 addresses. That step can be implemented and have support for it added later, whenever S is ready to serve clients beyond the IPv4 address space.
Does this make sense? If so, then it seems to show how IPv4-only clients could talk to IPv6 servers without modification. If not, then I'd love to see where I'm mistaken (I very well might be).