Robin:

Btw, I'm interested in "farming" first because growth rates suddenly increased by two orders of magnitude; by "farming" I mean whatever was the common local-in-time cause of that change. Writing was part of the cascade of changes, but it seems historically implausible to call writing the main cause of the increased growth rate. Professional specialization has more promise as a main cause, but it is still hard to see.

I don't think that this is what Eliezer is saying (and correct me if I'm wrong). What Robin seems to be inferring is a claim that writing or professional specialization was the cause of the 100-fold population increase. What Eliezer is actually arguing is that writing and professional specialization are more interesting than the population increase from the point of view of optimization processes, and the 100-fold population increase is merely incidental, even if it was a prerequisite. From the original post, my emphasis added:

Aha! - now we have writing. There's a significant invention, from the perspective of cumulative optimization by brains.

professional specialization... is a more significant jump in cumulative optimization than the gap between a hundred farmers and one hunter-gatherer pondering something.

The disagreement between Robin and Eliezer shouldn't be surprising, considering simple lapses in understanding such as these (and yes, I've seen others, but I'm not about to dig them up).