True, there are many civilizations and we all know about them like hindu claim that they are like 5000 year old, egypt, mayan, gandhara we all know about. May be muslims knew about china at that time but its not in discussion anywhere since its not of some significant importance. We know that many buzurgaan deen travel to different parts of the world to spread the message of Islam, may be some traveled to china but not that famous. Its same like zakir naik says that may be ram (hindu god) was a messenger of ALLAH but we cannot say for sure, even if that is it was for that time only.
I can't argue on the authenticity if its hadith or not, all I know that no body is sure so we should leave it to that without giving our own explanations as to how it came into existence.
