The din of learning hilchos hachag in preparation for the chag does not appear in hilchos talmud Torah -- it appears at the beginning of hilchos pesach. This suggests that it is a din in dinei Yom tov, not in limud hatorah. Nafka minah: there is no din of osek bmitzvah patur min hamitzvah by talmud Torah, but there is by other mitzvos.
Rashi in sukkah 25 writes that one who goes to a shiur to learn is patur from sukkah. Once upon a time in the past (http://divreichaim.blogspot.com/2015/09/talmud-torah-as-kiyum-of-simchas-yom-tov.html?m=1) we learned that the reason there is a ptur of oseik bmitzvah here for talmud Torah is because learning on Yom tov is a kiyum of simchas Yom tov. Rav Baruch Povarski says a different pshat. The case Rashi is speaking about is where the person is learning hilchos hachag. That's not a din in talmud Torah - it's a din in celebrating Yom tov.
Another nafka minah to this question is whether women have a chiyuv to learn hilchos hachag b'chag or not.