Talking about systemic racism is "impolite" in the way that talking about a rape culture is "impolite." Civilization is a system for perpetuating itself, and so it disallows systemic critique of its fundamental presuppositions. Demanding "politeness" is demanding that the conversation take place under a set of rules that disallow the arguments of one side.

I put polite in scare-quotes, because I think there is a real polite, built on respect for other human beings, which I hope we can recognize the value of, while recognizing that for a lot of people "polite" means not upsetting the status-quo.

This doesn't enter in to all the ways in which "polite" is demanded of PoC more than it's demanded of other people, the ways in which PoC can never be "polite" enough, and the ways in which "politeness" is not generally something you demand of the injured party

I encourage you to familiarize yourself with its parameters before stepping in. Here is a helpful, relatively brief summary