I mean, yeah, Howard Zinn "told on himself" several hundred pages before that quote from the afterword. His whole thing is that lots of American history is only told from one side and he wanted to popularize a different view. The point of that quote is that not everyone lays their biases out up front the way he did.

In your math example, yes, pi is a constant of nature that simply is. However, in choosing pi over Euler's number or the square root of -1 for your comment, you've made an implicit judgement about their relative importance. That's probably inconsequential for most math facts, but could be a lot more impactful for people presenting history or the news.