I would interpret the developers’ words as stretching only as far as, let’s say, 500 AD, at the very latest. I would actually be more conservative, and put it at squarely 0 AD, or even 200 BC. It doesn’t really matter, other than that the world was so very unified when the Roman Republic, the Mauryan Empire, the Qin dynasty and the Parthians ruled most of humanity. Technology didn’t change much to be honest, but still. And no Christianity too complicate things politically 
But again, that is only far, far in the future. Maybe in 2021 we’ll see “Ancient Cities II: Glory of Rome” What we can expect though early 2019 is turning nomadic hunter-gatherers using stone tools, to sedentary agriculturalists using very primitive copper tools. Which would mean starting in maybe 7000 BC and ending closer to 4000 BC, in the Mesopotamian case, or maybe 5000 to 2500 BC in Western Europe. The point is that bronze and writing should be either on the horizon or something very new and exciting nearing end-game, as things stand right now.
In games like Pharaoh, Caesar III, Zeus: Master of Olympus, Emperor: Rise of the Middle Kingdom, Tropico, Children of the Nile and Banished, time moves constantly, slowly enough to make the seasons noticeable and manageable (autumn harvests for example), but quickly enough that most games, most missions in the campaigns, would take several years, often a couple of decades. In Banished, my first great town is about 120 years old, and in Tropico, a “long” game takes sixty years if I remember correctly? The question is how to implement a speed that can reconcile the RPG elements we all seem to treasure, like following a single individual, born in that hut, over there, to those parents, over here, who grew up to be a true marksman with the bow, found himself a good wife, argued with her about the gods, had eight children, five of which died young, and now is buried under that mound over there… with the more epic scale of leading a tribe, founding a village, discover the secrets of the unknown, domesticate animals, farm crops, build great monuments, discover metallurgy, create copper tools, turn the village into a city, engage in trade, dominate the region, make war and fight battles… but if every year takes ten minutes, which is common in similar games… how can one do all that we described over, let’s say, tens of hours? That will probably be the trickiest question… if we “fast forward” we loose the RPG connection that we would have with our tribe, our village… but how else could we simulate what would be realistic: that the leaps of technology of the era took centuries, millennia…?
Edit: there is already a discussion on this topic: In-game Time in AC - #4 by Baikal
As for your thoughts on other cultures, conflicts and maps… I’m assuming that the world will consist of the city map, which would be our village and its vicinity, a few kilometres in length and width, full scale; and then a world, or region map, which would contain all other villages or cities that we know of, and with which we could trade, or fight and so on… but that seems to be going off-topic…