As I understand it, they were commandments to specific individuals, but at Sinai we were told that we are commanded in them by virtue of that same historical story (the proof being that Maimonides himself writes at the beginning of the laws of circumcision that the source of the commandment is the story of Abraham).
The sanctification of the month too, according to that Maimonides, was commanded to us at Sinai. Who says we were commanded in it before we were commanded in circumcision?
Rashi’s question is that it should have begun with commandments, and the commandment of circumcision and of being fruitful and multiplying appears before the commandment of “This month shall be for you.”
So even if “This month shall be for you” is the first commandment commanded to the entire Jewish people, the commandments to individuals still received general binding force at Mount Sinai…

Sanctifying the month is not a command to specific people but to the whole nation.
The other commandments became binding at Sinai. The fact that their origin is in an earlier command to specific individuals does not matter for this issue.
By the way, this is of course a very marginal point in the discussion. What matters in this question is the assumption that the essence of the Torah is the commandments. What exactly is the first commandment? It may be that Rabbi Yitzhak also agrees that circumcision or being fruitful and multiplying come earlier, but he took sanctifying the month as his example because it is an obviously public commandment.

Sanctifying the month also became binding at Sinai; it received new force there, as Maimonides mentions that all 613 commandments (including this one) were given at Sinai. I don’t see why to call it the first. But okay…
Thank you very much.

But it did not become public at Sinai.

What difference does it make whether it is public or private? If the claim is that the Torah is a book of Jewish law, then every instruction binding on the Jewish people should have been written in it, and circumcision is a commandment for the entire Jewish people even if the way it appears in the Torah is as a command to Abraham (by the way, it too has a public dimension: “This is My covenant, which you shall keep, between Me and you and your offspring after you: every male among you shall be circumcised”).

A commandment, by definition, is a public and ongoing command. When Moses was commanded to take a fiery serpent or to speak to the rock, that happened after the giving of the Torah, and nevertheless it is not counted.