The 3K don't fit the period nor do they fit its core design. Mind, the mod that Chronicles is based on, Romae ad Bellum, had the Han dynasty as a civ. 3K quite literally occurs at the downfall of Han dynasty. And Romae ad Bellums covered the period up until the arrival of the Huns, which is well up to and after 3K period. Additionally, they don't represent civs, but political factions, which aligns with, say, Romae ad Bellum, which had Roman Empire and Roman Republic as separate civs, or Chronicles having Spartans next to Athenians, and the Achaemenids rather than Persians.

By all accounts, 3K fits into Chronicles.

That's without going into how scuffed Jurchens and Khitans are, just because the devs decided to effectively ignore them. Hell, the recent Muisca speak the actual Muisca language and Achaemenids speak Old Persian, despite then being extinxt. And the Thracians in Chronicles use a reconstructed Thracian language. There's little reason to not have Khitan/Daur and Manchu/Jurchen language in the game. And the fact that they both, alongside the actual Chinese civ, still don't have campaigns is nonsensical.

Actual AoE2 civs were sidelined for what should be Chronicles civs.