I re-read the Peter Marks WaPo article and didn't find anything that specifically says that Ann and George would be the only two POC actors in the production -- just that they would be cast with actors of the same race. I don't want to belabor this point, but I feel like that's making an assumption based on something that was never stated. This was Todd's statement:
“They couldn’t agree on the specific casting choices that would lead to the richest-possible ‘All My Sons’ for 2019, and thus, Gregory Mosher has decided to step aside. We welcome Jack O’Brien as our director, and we’re looking forward to presenting his production of this Arthur Miller masterpiece this spring.”
For what it's worth, Curio Theatre Company in Philadelphia mounted a production of ALL MY SONS this past fall that featured actors of color in several roles, including both Ann and George. The Kellers were all played by white actors.