The article describes coexistence as both dual-stack and connectivity between single-stack IPv6 and single-stack IPv4 host. And that in the autor's opinion all the complexity is in the latter, not in the dual-stack
replyYou raise a good point that we also should't take dual- stack for granted. But I think the more precise question 'why not dual-stack as the only coexistence option' also seems like a good one, and one the article does not explore or even acknowledge
Dual-stack was the only coexistence option for a long time, until NAT64 came around. There were a whole bunch of attempts at compatibility, e.g. with "::1.1.1.1" and "::ffff:1.1.1.1" as IPv6 addresses, they just didn't go anywhere. (Well, not quite, the latter is in POSIX and in socket libraries around the planet. Doesn't leave the host though. At least it's not supposed to. I have some horror stories…)
replyNAT64 started happening because it solves real problems — large eyeball networks, particularly mobile phone networks, didn't want to pay for twice as large table sizes on their routers and twice the maintenance effort. So they made IPv6 end hosts capable of connecting to IPv4 systems. But this is 2010 era, IPv6 was ≈15 years old at that point!
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