The Brisker Rav (al hatorah, parshas Bo) points out that the concept of geirus must pre-date mattah torah because we find that there was an issur for a nochri to eat the first korban pesach. This is difficult to understand if one assumes that before mattan torah there only existed a single category of ben noach. Hoewever, based on R’ Shachter’s chiddush, one could explain that before mattan torah there existed geirus to join the mihspacha of the Avos; after mattan torah there was a higher level geirus to join Klal Yisrael.
This chiddush also explains the opinion of the MahaRaL who holds that we do not apply the principle of ‘ger shenitgayer k’katan shenolad’ to the geirus of mattan torah (this MaHaRaL is referenced in the Shav Shamytza’s introduction). Although mattan torah is the paradigm of geirus for joining the Jewish nation, it did not dissolve the bonds of Jewish mishpacha which already existed beforehand, so no issurei arayos were lifted.