Commentary for Sukkah 75:9

"Truly, they said 'A son can bless for his father.'" This here in the Gemara does not continue the topic the Mishna was discussing, but connects to the Mishna's issue of one who is cursed. In Berakhot 20b, there they wanted to prove from this Mishna here in Sukkah that women have an obligation for Birkat haMazon on a Torah law (de-Oraita) level, but that proof was rejected with the question, do minors also have an obligation on a de-Oraita level? So it was suggested there in Berakhot this case here in Sukkah could where one only ate a shiur (amount) that obligates on a Rabbinic level, and so in that case a woman could bless and a man's obligation could be fulfilled. However, in the Tosefta it says that a woman and minor do not fulfill the obligation of Birkat haMazon of the general public. This could be limited to the case where the other person eats an amount that generates a Torah law obligation. Therefore, it cannot be proved from here that women are obligated in Birkat haMazon on a Torah level, perhaps it is not Torah level because women do not combine to a Zimun, as it says in Berakhot 45a, that men can bless for them, but they cannot bless for men, so a man can include women in his Zimun, but a woman cannot include a man in a men's Zimun. Or perhaps it would be improper, similar to Megilah, where women are certainly included in the obligation, yet women cannot read the Megilah for men according to the understanding of the Baal Halakhot Gedolot.