hehe, I am aware of this, but, as total war in general is a "what if" scenario, one can assume Khwarezm and the Byzantine empire would have kept updating their militaries if they had survived/avoided decline.
Now here's a crazy idea: One could develop an apache script (unlocking units based on setlements captured/units destroyed) where the Byzantines get a different roster depending on how well they're doing in the campaign. Their units go down the toilet in the long run if they get slowly grinded to dust by the turks as it was in history, or get new flashy units if they manage to repel the invasions and conquer long lost settlements in the levant or somesuch