NOT WASTING TIME
In light of this explanation of the impurity of a woman who gave birth, we can now understand Rashi’s explanation of the word, “He shall offer it — The one which comes to atone, upon it does the purity depend” — meaning that the woman’s purification hinges on the sin-offering that brings atonement, and not on the burnt-offering.
It specifically hinges on the atonement sacrifice because the impurity stems from something that must be atoned for. Therefore, the purification of this impurity depends upon her receiving atonement.
In light of the above explanation, we can better understand and appreciate Rashi’s subsequent comment. Immediately after his above-mentioned explanation of “He shall offer it” (where he emphasizes that the purification of a woman who gives birth is connected with her receiving atonement), he then says, “and she shall become purified — this implies that up to this point (she) is called impure”: Now that we know that the impurity of a woman who gives birth is an offshoot of the sin of the Tree of Knowledge, and her purification depends upon her receiving atonement — it follows that “up to this point (she) is called impure”: Even after the impurity {of giving birth} is removed, as long as she has not received atonement, she is still “called impure.”
This also clarifies what practical difference there is (even according to pshat) that “up to this point (she) is called impure”: If the prohibition against her partaking of sacrificial foods were merely because she was still not entirely pure, it would be quite plausible that she wouldn’t feel any urgency to bring a sacrifice to allow her to eat sacrificial foods. There is no obligation for a Jew to make sure that they are pure enough to eat sacrificial foods (except before the Festivals, when a person must indeed ensure that he is pure on the Festival).
However, now that we understand that she is “called impure” because she still lacks atonement, it becomes clear that she must hasten to obtain atonement as soon as possible.
