Someone had carried her in.
Borne her in.
is an incomplete sentence. It has an object (her) but is missing the subject. It is also missing part of the verb. The simple past tense of "to bear" (meaning to carry) is "bore", not borne, "It bore her in" would be the correct simple past tense.
The missing parts of the sentence are implied in the previous sentence.
Someone had pulled her up.
Here the subject is 'someone', and the verb is 'had pulled'. The next sentence is a fragment, but we can understand that it repeats, as a literary device, the subject and auxiliary verb of the previous sentence.
Someone had pulled her up. [Someone had] borne her in.
With the auxiliary verb 'had', the conjugation 'borne' makes sense, and we know that the subject of the sentence is 'someone' - and likely the same someone as before.
The car she is in is falling, or crashing. She reaches for the door, even though it is impossible to escape (she is defying her inevitable death). Someone reached into the car, pulled her up and out of it, bore her weight as they lifted her. She ends alive, but held by the person who rescued her, suspended off her feet.
The text is retelling something that happened, so it is all past tense, but most of it is simple past tense. The two sentences in question, though, shift to past perfect to emphasize the sudden, unexpectedness of the rescue being performed and completed before the narrative returns to the simple past tense.