We could be 100% black (which we're not) and it wouldn't matter. We could still say that we're white. What would you do about it? Post memes? Quietly agree amongst yourselves that we aren't? You certainly couldn't fight the strongest nation on earth over it. Is it that important to you to prove us wrong? We can wallow in our ignorance for eternity and there's nothing you could do to force us to believe that you're right. Our beliefs (that we're 100% white, which many of us are) trump your petty "truths" (that we aren't.)
The truth doesn't matter as long as we ignore it, and there's nothing you can do to stop it.

What this really is, though, isn't even about being <100% white. Because if that was the case, Europe would be under the bus too. What it really is is about insecurity. Europe is falling down the same slope it thinks we are, and it's trying to justify its increasing inferiority by arguing that we're worse. It's okay, France. Nobody will worry about whiteness anymore once your 12% becomes the 100%