I don’t think it’s that, as the same update brought unique castles, unique UU skins and regional monks skins, which feels like a similar amount of work. Which they didn’t need to do for that specific update. Also, still having the same medieval units that are already the same from the Aztecs to the Japanese, isn’t as weird as having them for Ancient Greece.
I’d rather say it’s first some clumsy way of courting the chinese market into the game. “Hello fellow Chinese customers, we have that thing from your history that’s massively popular and you can bring those heroes against the Mongols the Japanese and whatnot”.
Don’t get me wrong I’m all for more architectural styles and I’d add :
- Steppes : Mongols Khitans Cumans Huns
- Medieval Chinese : Chinese Koreans (*) Jurchens Vietnamese
- Old Chinese : 3K
To split up the east asian style (I’d add some for Europe too). About the Koreans I’m not sure if keeping the current mostly-japanese style wouldn’t be better, hence the asterisk I put for them.