Lots of interesting details around the beginning of writing systems but it seems to overdramatise the conditions and role of Sumer in kickstarting civilization as we know it?
> the original organized, literate, urban culture was produced by a far crueler and more challenging environment than either of those (Indus, Nile)
Egypt and the Nile don't seem particularly different as an environment. Dry desert with regular river floodings etc.
> The Sumerians invented kingship, priesthood, diplomacy, law, and war.
Of those, only law sounds like a legitimate priority claim, in the sense of having writen, thus fixed, laws rather than whatever the potentate of the day feels like declaring as being the law today.
> Egypt and the Nile don't seem particularly different as an environment. Dry desert with regular river floodings etc.
The Nile flood was extremely benign compared to Mesopotamia. You can set your calendar by it (and the Egyptians did), you can measure the height of the flood for a few years and be very confident that the flood of the next year will be similar to the ones of the past few years. And most crucially, the Nile flows in a deep river valley, with fairly steep sides, meaning that you can build your houses and granaries within comfortable walking distance of the flood plain and still be confident that you will be safe from the floods.
In contrast, the floods of Tigris and Euphrates were extremely unpredictable in both timing and strength, and the flood plain was so large that the entire civilizations working it had to live in it.
This resulted in very different outlooks, most visible in how they viewed their river gods. The Egyptian ones were friendly, the Sumerian ones were capricious and cruel.
That phrase is rather imprecise.
What can be attributed to the Sumerian cities is the first appearance of several professions which are not directly productive and which have remained important until today.
By profession, I mean an activity that was all that a certain human did in order to obtain all the resources required for living. Before Sumer, those activities were done by certain people, but only occasionally, besides their main craft, e.g. a shaman was likely to also act as a medical doctor when needed and a chief of tribe was likely to also judge conflicts.
Among the professions that have first appeared in Sumer during the third millennium BC, are: judge, medical doctor, accountant, musician, prostitute.
The equivalent of priests and kings, though only at smaller scales, e.g. for a tribe or a confederation of tribes, have certainly existed much earlier than Sumer and Egypt. "Chief" is likely to be the oldest profession, as that exists at many social animals, including apes.