In an essay on her writerly influences, Oates characterizes this description as a "terse encomium." In her essay on Cormac McCarthy, Oates mentions the reference again, this time addressing the apparent contradiction of the encomium's plural subject by suggesting that it is "as if the singular Dilsey were in fact multiple, emblematic."
Oates is trying too hard here, and not making much sense.
In fact, the final line of the Appendix, "They endured," applies to all four of the black people listed—TP, Frony, Luster, and Dilsey. Dilsey alone among the characters discussed in the Appendix (some quite brutally) is granted the dignity of not being summed up at all—as if this solid, powerful woman cannot be captured in words.
Oates' confusion arises from the formatting of the Appendix. The line "They endured" comes directly below Dilsey's name, and she is the last black character listed. But every other character's description begins directly after their name, on the same line. So it may appear that "They endured" is a description of Dilsey, but upon comparison with the other characters it is clear that Dilsey's name is simply left to stand for itself, and that "They endured" is a description of all of the black characters who survive alongside the self-destructing Compsons.