Shakespeare appropriates Danish culture in Hamlet, Italian culture in Romeo and Juliet, and Scottish culture in Macbeth. So let's burn these works and fire all the English teachers.
"...my opinion on cultural appropriation of my own culture wasn’t valid" No, your opinion was disagreed with. The only opinion being invalidated is the one being censored.
“It seems like a continual blotting out of our right to our own voices.” "...white writers don’t get out of the way and make space for the multitude of stories to be told by those who aren’t white and middle class.” 
Is there a shortage of words, or paper? Other people writing about your culture doesn't stop you from doing so yourself. All this is just a call for race-based censorship, and from a "Writers' Union," no less.