That statement gives off the impression that Nubia was a moderately well-known character before this year.
The concept of Nubia being Diana's twin gives the immediate impression that they are equal. And yes, it does come with a guttural responds of "token black character", but unfortunately when a black character is introduced into any franchise, that is a response that is heard.
If Nubia was introduced, not as Diana's twin sister, but still a woman equal to Diana and earning the title of Wonder Woman, the response that she was a "token black character" would still be felt.
The criticism that making Nubia Diana's twin would restrict her to being a character defined by that title, would, I think, depend on the execution. Nubia really hasn't had many chances to be a character yet. She's appeared in more issues of
Wonder Woman as a character who
wasn't Diana's twin, and it hasn't done her any favors either.
I fell though, by having Nubia's original debut be Diana's twin sister, that has at least paved the way for her to be given a bigger roll in the Wonder Woman mythos than she is presented with now. It has at least paved the way for Nubia to be given powers on par with Diana's.