Myers is attacking Kurzweil on the assumption that Kurzweil is actually proposing that the brain's structure should be reverse-engineesed from DNA, which would be intractably hard. Only Kurzweil isn't saying this anywhere. Instead, he seems to be using DNA as a measure for the amount of irreducible complexity that needs to go into a system that will end up with the complexity of a human brain.
Basically, we have no idea where we will get the AI source code we can actually do something with, but we have some reason to believe that the most concise version of the source code won't contain more data than the human genome.
The rough intuition might be if we wanted to simulate brain in the caricatured reverse-engineer-the-DNA way, we'd need an impossible computer that could simulate years worth of exact quantum-level physics in a cubic meter of space, but the human DNA and basic cellular machinery for hosting it would be the only seriously difficult bits to stick in there, the rest would just be simple chemicals and the impossibly detailed physics simulation.
I guess the analogy then is that we make the AI source code (which we don't know how to write yet), which is supposed to end up at most around the length of a human DNA, but which can run sensibly in existing hardware. Then, deterministic processing entirely unlike the immensely difficult-to-compute protein folding from DNA will make this code instantiate a working AI somewhere at the level of a newborn human baby, in the same way as the genome initiates protein-folding based processes that makes a single cell grow into a human baby given little more than nutrients and shelter from the external physical environment.
So it doesn't seem like a really strong statement of overoptimism. It's basically just saying that the human brain doesn't seem to require a mystifyingly immense amount of initial information to form, but instead something that can be quantified and very roughly compared with already existing software projects. I'd still guess it might take a little more than the ten years to come up with any sensible code with hope of growing into an AI though.