It's not just about population - China by the 1800s had a population approaching half a billion, but they had almost no industrialization compare to the Europeans at the time.

>Were they really there? I thought they made it to the point of being equivalent to the 15th century but never got any further technologically.

In terms of metallurgical technology, they were closer to 13th century Europe-level. Just looking at iron production, the Romans of the late Empire were still running dinky bloomery furnaces to smelt iron- not much different from the ones their forefathers were operating when their city was found.

Historic and archeological evidence suggest the Romans never produced more than a hundred pounds of iron from their furnaces, while European smelters by the time of the 1200s were already producing nearly a ton of iron per run from their larger furnaces (which were more efficient in terms of manpower and fuel consumption). By the 1300s, they were able to produce a few tons of iron from each furnace operation, which spelled out to a greater and cheap iron supply that helped pushed the adoption of plate armour, and then gun artillery.