It is true that people who self-identify as poly tend to be better well off, white and well-educated. That's been my personal experience to some degree. My local community attempts to be more diverse in both race and class.
However, I suggest that there may be significant numbers of people who are in effect poly but don't use that term. Some of those people are likely to be minorities either in class or race. I think it may be somewhat similar to men who have sex with men but don't identify as gay, in part because gay is seen as a white identity. A similar dynamic may be at work here too.