I think in general a "mixer" at a con is exactly along the lines of what you're thinking, it's a small party meant for small social groups to gather at a larger event. I guess my question, given the tone of the linked article, is whether the writer was truly looking for a white, straight, male mixer, or if they were just looking to stir up trouble because of the other mixers being exclusive? I can see why women, or black people, or LGBTQ people, or any other minority, might be looking to have a small event where they don't feel like they have to look over their shoulder. I'm not sure why a white dude at a comic show would ever feel that way, as they/we (again, straight, white male dude here) overwhelmingly represent the majority at these events. But as for whether or not the con itself is sponsoring discriminatory events, that could very well be a valid legal question, and I'd have to defer to the legal eagles on the boards to weigh in on whether or not that is kosher.