The real wall sections were built around 5th Century BC. Since the Hyborian Age is set 10,000 years before Christ, did you make a connection about the wall and the real world wall?
No they weren't. The "Great Wall of China" has always been a moveable feast, depending on which one you mean. There was a Great Wall (the first one recorded as being built as a single unit) in the 3rd Century BC (Qin Dynasty), but little or nothing of it remains today. There were earlier fortifications of the northern borders of various Chinese states back to the 6th century BC, but their course and extent isn't certain. There followed a succession of similar walls as the Chinese border moved north and south, culminating with the 16th Century (Ming Dynasty) wall which stands today, a good distance south of its Qin forbear's former line. If Hyborian Age khitai had a Great Wall, it fits very neatly into the long history of similar walls built and abandoned over the centuries.