Nothing could be clearer: every manner of commandment came through Moses at Mount Sinai. So why does tradition challenge this statement by saying in a number of places that Abraham, Isaac and Jacob kept the Torah long before Moses and Mount Sinai? Could there have been a pre-Mosaic Torah?
It seems there was. Cain was punished for murder generations before the Ten Commandments. In rabbinic tradition, the generation of the flood were punished for robbery, the men of Sodom for lack of charity, and other generations for other sins. So even without the Torah, there were obligations that had to be honoured.
What these proofs have in common is that they deal with basic moral attitudes which even the earliest generations could and should have realised were necessary.
At the time of Moses, God strengthened these moral duties and built around them an entire code. But the moral mitzvot were obligatory from the time of Adam onwards.