Well, then we have now 2 cavalry civilizations. Actually I didn't think they would include the mongols after scythia. However, since Korea is also included,
maybe there is a scenario planned which needs them.
The Mongols were always likely, and if they get a horse archer it will almost certainly be a knight replacement (or at least a unique unit of the medieval era) to fit their historical period. Scythia’s Saka archers come much earlier.
Civ V had the Huns and the Mongols, and even Arabia had camel archers which behaved in a very similar way. They’re not averse to having multiple cavalry archer units.