How about the human understanding of murder, theft, adultery, etc. prior to the Noachide laws and the Ten Commandments? Were these not sins until formalized by God? That is that from Adam to Noah no man could be condemned for these sins.
Are you saying that Noah and Moses received new revelation not previously known to mankind? How would this affect the sinning by those outside of the Israelite context, i.e. Chinese, Egyptians, the whole rest of the world. Are they not under condemnation for sin?
I think that I explained the Natural Law very badly if you ask this.
Yes, any form of offending God, including murder, theft and adultery are violations of Natural Law and so are sins and have been even before the flood. The cause of the flood is God's regret of making man in the face of the sins of man (Gen 6:5f).
It is because Natural Law is sufficiently clear that the Chinese, Egyptians, the whole rest of the world indeed are under condemnation for sin.
The role of written revelations of biblical laws is to shape a Kingdom of God, step by step, starting with the Jews and then including the Christian Gentiles. It builds upon the Natural Law. It doesn't excuse anyone who did not receive the written revelation (again, Romans 2, especially 12-15).