The Problem With Shriekers III
The third problem is the world's Tech Level. There is no outright admission of the TL of the Great Retreat, which is nevertheless clearly stated as the last bastion of civilization on the planet.
T5 describes TL1 as "The culture, which already uses simple tools (rocks; clubs) uses them to make other, more sophisticated tools.". Since the Shriekers make walking legs and animal harnesses, that puts them at TL 1.
The case for TL 1.6. TL 1.6 is described as having kingdoms, cities, and galleys. We know for sure that the Sun
Worshipper empire had these, and so was TL 1.6. We also know that the Great Retreat was originally part of a complex government of cities, so they are reasonably TL 1.6 at least. So, TL 1.
The case for TL 2. TL 2 is described as "Age of Sail". It gives anatomy, algebra, printing, and of course sail as benchmarks. The only item here we know for sure is anatomy, due to the description of how they perform surgery on their legs in order to fit them for higher-walking prostheses. That presumes a reasonable level of anatomy. However, I have no indication of printing, higher math, or sail technology. It's not that it's unreasonable; it's that we have no data.
So they have anatomy, writing, some metallurgy, a kingdom strong enough to expand, and withstand and influence the barbarian tribes of the plains. What it would take for the Great Retreat to be TL 2 is algebra and print - probably a printing press. Then at some point they'd have to figure out sail, although the Great Retreat is not next to the Great Sea.
So, it is my conclusion that, though 567-908 is reasonably TL 1, elevating it to TL 2 would be doable and, I would think, is a plot hook on a platter.