I'm in the same boat as the parent poster in terms of time spent on 4chan and 8chan, and I honestly can't tell if you're being sarcastic. I understand the nature of irony on the chans, and that a lot of "-ist" comments are "kidding" (to varying degrees of success in terms of comedy and responsibility for the message one is putting out there), but:
1. To be so sure about the numbers seems presumptuous.
I know several people who lurk and post on /pol/, and it is an ethnically diverse group from my perspective. I've heard this sentiment echoed by others.
Of course, if you live in a very white place, you probably can't tell either way, even if you know a lot of channers.
Maybe there are a lot of unironic black and native-mixed neo-nazis, but I'm not so sure myself.
Keep in mind that it's still mostly ironic.