My memory of the issue (which could be wrong) was that Miller's concern was equally about whether we could believe that a white man and a black man would have designed airplanes together during the War years - or at least that if they had, it would have been talked about in a way that the play doesn't currently allow.
I do agree with you that the solution they found is still a bit hazy. I like to believe that Miller was advocating for more actors of color, so that the play would be truly diverse, rather than a set token amount of two, and that this was a compromise that did its best to please everyone. A true color blind cast would be closer to no more than half of the cast being white. But that's a lot to ask the Roundabout (hence the letter from the anonymous staffer that made the rounds a few months ago).