I remember reading somewhere that Gilgabro's Akkadian is of the Babylonian dialect. Which, if true, means that Sumeria is effectively an Akkadian blob, using Assyrian clothes, Babylonian speech, and a universal Akkadian structure in the Ziggurat.
From a game design perspective, I think this is borderline genius. Gilgamesh was a folk hero in Akkadian, Assyrian, and Babylonian culture, and Assyria and Babylon especially revered and mimicked Sumeria. Despite being ethnically distinct, Sumeria really was the ancestral root of the Mesopotamian burgeoning and is an adequate proxy for all three civs. In this respect Sumeria is functioning as an Akkadian blob, repping the entire region for that period by assimilating the stronger aspects from prior Civ incarnations of Babylon and Assyria and eliminating the problem of having to design two distinct and fleshed out Mesopotamian civs. A problem which gets harder with every installment as every aspect of art design becomes deeper and more nuanced in service of distinguishing civs as different personalities.
On top of that, Sumeria holds its own niche in the game as the ultimate beginner civ, which I find quite fitting for a civ which "invented the wheel." It has perhaps the most basic martial and scientific bonuses in VI and is extremely easy to play. And the "I did it first" attitude the design of Sumeria takes toward Assyria and Babylon drives this theme home even further.
So, I understand why historians might be disappointed that a) Sumeria is not Sumeria and b) there probably won't be Assyria or Babylon in VI. But I think as a matter of consolidation I think this was extremely responsible game design. It's not offensively generalist like Polynesia in V; rather it is deliberately painting Sumeria through the eyes of later Akkadian civs. It implicitly encourages the same "cultural throughline" interpretation of the Mesopotamian region in the same way that Chandragupta and Eleanor and subsequent alt leaders expressly emphasize. Perhaps a bit obtuse, but on the spectrum of historical inaccuracy I think this falls under thought-provoking artistic license more than it does blatant or lazy misrepresentation.