Does the Torah of our day still reflect God’s will?

peace,
I heard the rabbi claim that most of the Toshab’a did not descend from Sinai but rather developed later, towards the end of the Second Temple period. Ancient halacha was also largely a kind of interpretation of the Toshab’a, in contrast to today’s Toshab’a. And it served as a kind of conduit for the transmission of rumor and tradition. In contrast to human innovations as seen in Tractate Avot.
So I wanted to ask, if there are such major changes between today’s Torah and what God revealed at Sinai. Why does the Rabbi assume that the Torah we have today still reflects God’s will?!
I would be happy if the Rabbi could answer….