Based on the Maharsha’s explanation, we can understand why the Rishonim cite Rabbi Shimon bar Yochai’s solution as to why a postpartum woman must bring a korban.
The Gemara (Nedarim 4b) says that the korban chatas that a woman brings after she gives birth is not brought for atonement purposes. The Ran (4a) explains that this conforms with Rabbi Shimon bar Yochai’s opinion as well. This is because even Rabbi Shimon bar Yochai agrees that the main reason why she brings this korban is not for atonement purposes. He proves this from the fact that a woman who is absolutely certain that she did not recite an oath must still bring the korban.
Additionally, the Gemara (Kerisus 26a) says that even Rabbi Shimon bar Yochai agrees that the korban chatas that a postpartum woman brings is not brought for reasons of atonement but rather to allow her to eat kadshim. Rashi explains that the pain she experienced during labor atoned for her sins. Accordingly, she is no longer required to bring a korban chatas.
Based on this, we can understand that Rabbi Shimon bar Yochai’s given reason for the korban chatas was only meant to serve as a partial explanation. It is therefore accepted – even if the Gemara questioned it.