I completely agree with the principle of that point, but the issue I have is that we'll never get past this (racism, sexism, homophobia, any of it) if we always have to tread on eggshells because of the past. Millions of people who saw that cartoon just took it at face value, a person being lampooned for having a tantrum, but they're then being told that this isn't the case, that the cartoon is racist because it - vaguely - resembles cartoons which were deliberately racist nigh on 100 years ago. People then feel that black people have to be treated differently, and a division is created where previously there was none.
Look at how some of the arguments for it being racist are framed. There's a quote which somebody posted earlier which basically says the artist has tried to portray a black person as an animal and uncivilised. Is that really what you see when you look at it? I don't, I don't think that's what it's going for at all, I think it's trying to portray Serena Williams as a child throwing a tantrum, not an uncivilised animal. The article's reaching for something that's simply not there. Another newspaper piece claims the cartoon depicts her "wearing a skirt that might remind some of the banana skirt worn by the late Josephine Baker". But the skirt she's wearing in the picture is the skirt she was wearing in the game. The writer is actually trying to put forward an argument that drawing Serena in her own choice of clothes is racist. That is absolutely scandalous in my opinion, and again it means that people become scared to ever say/draw/think anything in case it gets twisted into something racist. That's not a healthy way for society to function.
I'm not saying we pretend the past never happened, and i'm not saying the bloke who drew the cartoon isn't racist - he could be a fully paid up KKK member for all I know - just that making a massive song and dance about things that could be construed as whateverist when looked at in a certain way and taking into account things that happened a long time ago, prevents the sort of societal progression that we've seen in other areas.