"black people cannot be racist to white people in the same way that whites have been to them for centuries"
I think what you mean is "black people, living in the United States in the situation that obtains currently, cannot be racist..." etc. And it's fairly difficult to disagree with that.
But as you point out, this is about systems, not the nature of people. It's surely not the case that black people could never ever conceivably be racist, in some other country or in some future; it's that the current society in the US (and to some extent in other countries too) isn't one where they (as a group) could be racist toward white people.
Nice points about affirmative action, incidentally. I too used to be sceptical about it, on the usual basis, but a few years ago I started to grasp that feeding the starving didn't have to mean starving the well-fed. It's not a panacea and it needs to be applied carefully to make a meaningful difference, but ultimately it can have substantial positives that far outweigh some mostly theoretical negatives.