Chiyuv D'Rabanan Being Motzei A D'Oraisa: An Interesting Mordechai

In preparing for this week's shiur (which is about whether a katan who becomes bar mitzvah in the middle of sefira has a chiyuv to count) I came across an interesting Mordechai in Megillah (798) that is quoted by the Minchas Chinuch (Parshas Emor).

The Mordechai asks how can I blind person who only has a chiyuv d'rabanan be motzei his children in kiddush since they have a chiyuv d'oreisa. He brings a rayah that this works from a gemara in Berachos. The gemara says that one can be m'kabeil Shabbos early and make kiddush. The Mordechai understands that your chiyuv kiddush before tzais is only d'rabanan. Nevertheless, you will not have to make kiddush again after tzais even though your chiyuv kiddush which will turn into a chiyuv d'oreisa after tzais. Therefore, we see from here that a d'rabanan can be motzei a d'oreisa and a blind person can make kiddush for his children.

My problem with this Mordechai is that it seems to me the two cases are not similar. Unless what the Mordechai means is that if the case in Berachos is okay (kiddush before tzais) then certainly the blind person case is also fine.