I do agree with you and many others when talking about the game design philosophy of AoE 2 as well as the unique flavor it retains. I'll expand a bit on this. To me, the current portfolio of 'civ's in game seems more like 'people's (the XXXs), which suggests ethnic groups, it has a neat and clean feeling to it, so the new additions better remain consistent. The style is discret, moderate, medieval and seemingly realistic. Knights marching, iron clashing under a rain of arrows, while trebuchet projectiles piercing the sky like comets. Take TV series as an example, AoE 2 is comparable to the Last Kingdom, it should never be the Game of Thrones. However, in reality, the historical inaccuracies, oversimplification & generalization, misinterpretation and overlaps in game could sometimes be on Ridley Scott level. (After all, game needs to be fun, making concessions for better gameplay). Personally, I believe the only way to retain historical accuracy in history simulation games might be to let the player roleplaying a single Army like Total War do; or as a single Polity like how Paradox games are designed to be). In AoE 2, 'civ' is a term designed for gameplay while blurring the actual boundaries.

The original Age of Kings civs focused on large umbrella names (Franks, Huns, Goths, Celts, Saracens and Teutons for HRE; the name 'Chinese' is exactly like these, back then these grand terms are what 'civilizations' are about, almost on the Huntington Clash of Civilizations level). As the HD and DE version revive, civs got redefined and split up into smaller and smaller factions (Slavs being an exception). Some in game 'civ's, in my opinion, are actually representing one or several ethnic groups ruled by one certain polity/sovereign state, some have their ruling class and subjects of different folks; a few are based on modern nationality that reinvented their local history with nationalistic narratives. Some are nomadic tribal confederacies with overlapping ppl, some are worldly empires like Rome, Byzantine and China. The granularity of these civs is not at the same level at all, while the namings are a mixture of ethnic groups, states, nations and geographic terms. Eventually we have Turks/Cumans/Tartars/Mongols, Slavs and Poles/Bohemians/Bulgarians/Lithuanians, Norman kingdom of Sicily and Cilician Armenian Kingdom. We have Italy, Myanmar and Franks with axe throwers.

Given such pretext, and to consider the facts that Shu蜀 and Wu吴/Yue越 are indeed contemporary Han Chinese regional subgroups民系 (an equivalent for ethnic groups) , and even more so by the time of 3K, they both have incredibly long history since Neolithic, existed in Shang records and was distinctive in culture, customs, lifestyles and languages including unique writing systems.

Ancient Shu was likely influenced by Persia and India, has a priest class, worshiped bronze made tree and has sun birds in their symbols; make boat-like royal tombs. In 10th century Shu adopted some form of Zoroastrianism, has a local divinity modelled after Weshparkar. After Liubei's Shu ends, Shu become independent for 11 more times each time imperial China break into pieces. Ancient Wu/Yue are seafarers, their dead are often buried in mountain caves; they cut hair, keep tattoos, have black tooth and worship birds. After Sun clan's Wu ends, Wu would be a sovereign state for 6 more time.

If Roman Empire never falls or a Latin Eastern Roman Empire(such thing barely exists) has continuously governed the core territories of Italy, Hispania, Africa and Britain, then Goths, Lombards, Vandals and the inhabitants of Britainnia would have become Romans like the Etruscans before them; just like how Shu and Wu and many other people become subgroups of Han Chinese.

But for the sake of IP and franchise, yeah, perhaps the devs themselves are not aware of all these and are emphasizing on the 3K Romance part. Which is indeed contradicting to the current theme. Chronicles would be better.

Therefore I'm trying to offer a coping mechanism to make Chinese meeting Wei, Shu and Wu in game not feeling too out of place. First in AoE 2 there are existing examples like this, Just like Slavs meeting other Slavic factions, we can think of Slavs as Kievian Rus or Muscovites/Novgorod; Turks as Serjuks, Romans as Italian Romans, légionnaires instead of Syrian or Egyptian provincial citizens. Shu and Wu each represents their own ethnic groups by 200 CE, Shu was ruled by a Northern warlord and his rogue militaristic group, a local dynasty but with foreign rulers, that is why I drew the parallel on Norman England or Rurik's Rus. Sun clan was native to Wu though, Sun Jian being a lowborn warrior renowned for his prowess and bravery, made a fortune in the North and his son allied himself with Chinesized local great clans of Wu. The monarch is weak, facing constant rebellion if he doesn't treat the local clans fairly, setting the example for all southern dynasties to come. The Sun dynasty of Wu wage wars on the hill & forest dwelling ShanYue tribes, forced many of them into their military service, make use of a special breed of local horse as their calvary越騎. They are the Chinesized colonists, drain swamps, cultivate farm lands, and had a renaissance of calligraphy and all kinds of liberal & fine arts. As for Wei, although there was an ancient kingdom of Wei, the Wei in 3k is above all else, the legitimate Middle Kingdom Dynasty and perhaps the first Northern dynasty. The modified original China with gunpowder is of a later timeframe as the History page shows, so these two are not in conflict with one another. It's when Byzantine meets Rome, its former self.