I read that last week online and thought it was really well done.
Yes, I did, too--thought it was well done, I mean. I thought he handled well both the novel itself and the suspicions about its provenance.
NYT columnist Joe Nocera today wrote something to the effect that there's no dissonance between the two Atticus Finchs, because both are fictional creations. I think it's more interesting than that. I assume the character is based at least in part on Harper Lee's own father, and her own feelings about him. And like you, I don't see anything astonishing about the idea that a lawyer could both respect law and justice and also be racist, especially in a small town in the 1930s South.
Gopnik says pretty much the same thing, or something similar, IIRC.
I've kind of wondered, too, whether some of those people who are so upset about the apparent "difference" in Atticus don't have their idea of Atticus influenced as much by Gregory Peck in the movie as by the book.

I sort of had to laugh at Gopnik's comment about
Mockingbird being on eighth grade curricula. I guess I'm just too old because it sure wasn't on my eighth grade curriculum! All I remember reading in eighth grade was
Fail-Safe. My English teacher was a young hottie with curly brown hair named Jim Hontz--oh, never mind!

Gopnik's essay also makes we want to know more about the Southern Agrarians. Obviously I recognized the name Robert Penn Warren, and I've heard of John Crowe Ransom, but I don't know anything about him.
Edit to Add:Here's the Wikipedia article on John Crowe Ransom. The second paragraph under "Career" discusses the Southern Agrarians.
https://en.wikipedia.org/wiki/John_Crowe_RansomAnd on the Southern Agrarians:
https://en.wikipedia.org/wiki/Southern_Agrarians