I can't answer what percentage of Jews accepted the binding of Talmud, at the time, but it is binding on all. See the next part, 37 where he writes:
הוֹאִיל וְכָל אוֹתָן הַדְּבָרִים שֶׁבַּתַּלְמוּד הִסְכִּימוּ עֲלֵיהֶם
כָּל יִשְׂרָאֵל, וְאוֹתָן הַחֲכָמִים שֶׁהִתְקִינוּ אוֹ שֶׁגָּזְרוּ אוֹ
שֶׁהִנְהִיגוּ אוֹ שֶׁדָּנוּ דִּין וְלִמְּדוּ שֶׁהַמִּשְׁפָּט כָּךְ
הוּא הֶם כָּל חַכְמֵי יִשְׂרָאֵל אוֹ רֻבָּן, וְהֶם שֶׁשָּׁמְעוּ
הַקַּבָּלָה בְּעִיקְרֵי הַתּוֹרָה כֻּלָּהּ, אִישׁ מִפִּי אִישׁ עַד
The [Talmudic] Sages who established ordinances and decrees, put
customs into practice, arrived at legal decisions, and taught [the
people] concerning certain judgments represented the totality of the
Sages of Israel or, at least, the majority of them. They received the
tradition regarding the fundamental aspects of the Torah in its
entirety, generation after generation, [in a chain beginning with]
Moses, our teacher.
Meaning, the Talmud represents the unbroken total chain since Moshe at Sinai. Practically the whole point of this introduction to the Mishna Torah is to demonstrate this - the Torah and it's explanation were given over to Moshe, and passed down through the authority of every generation to the next authority (which is exactly what Hashem commanded us to do), and we have all the names (big names) and courts, and who learned from whom, right up until the Talmud, which was the last step until the real exile began and the period of the Geonim. There was nothing else representing the authority of Torah, that was it.
The point of this introduction is to prove to you that the Talmud is what we have left of Moshe Rabbeinu's teaching, and the Rabbis even to this day are faithfully transmitting his teachings on his authority, and there is none other.