Night. This is also apropos for Parshat Yitro
How did Taryag Come about?
I see this as a machloqet between Rashi and Rambam re: the last Mishnah in Gid Hanasheh
According to Rashi aiui, Mitzvot accumulated up through Sinai - which was a "makkeh bepatish" to what was already evolving into Torah.
As per Rambam - Sinai was a clean-break from the past, and any pre-Sinai "proto-Torah" was erased and our Torah was established anew in toto in one fell swoop.
What forced the Rambam's Hand? I feel the Rambam was forced to take this position because any Torah NOT given via Moshe Rabbeinu was inherently revisable - EG mitzvot given to Avraham or Yaaqov COULD be trumped by a greater navi - which is unthinkable to the Rambam. [See the Rambam's dogmatic take on "Lo qam k'Moshe"]
OTOH Rashi was not so concerned - and you might also say that Moshe's confirmation @ Sinai removed that concern anyway.
This dovetails with the "split" between Ashk'nazic approach to Halachah and the Andalusian-Sephardic approach
Ashk'naz - Torah is treated like "common law" and is based upon precedent, etc.
Batrai extends beyond Rav Ashi. L'mashal In USA constitutional law includes court rulings, NOT just the document and its amendments. And so
A lot like "oqeir harim"
Rambam holds Torah is legislated
First @ Sinai
Then later by Bd Hagadol in Yerushalayim [see MT hilchot Mamrim 1:1
Then Hasimat Hatalmud
Rav Ashi is final batrai.
Etc. [See haqdamah to MT and to Mishnayyot]
Rambam's Gestalt is Top-down. Authoritarian.
More Sinai like
EG You don't see a lot of "yeish omrims" in Mishnah Torah, because there is ONE way [or the highway! :-)]
The Shulchan Aruch OTOH drew from both schools, and that's where I often quibble because this eclectic method leaves a murky legacy at times about brachot etc. Sometimes SA waxes Maimonidean, [EG opposing Hanotein laya'eif ko'ach] at others Tosafistic [EG advocating L'hadlliq ner shel hanukkah in shul.]
. The Rema's virtual "rebbe" on Talmud was AFAIK the Mordechai, who did use the Rif as his core but was not exposed to Spanish rigidities..
These 2 schools play out even early in Jewish Halachic history, EG BS vs. BH
R Yihsma'el vs. R Aqiva, etc.
And might even go back even earlier.