Earlier quoted context omitted.
I think that the right has moved the Overton Window so much to the right that they see things that most of the world would see as mainstream (Black Lives Matter, Abortion, Medicare for All, Defund the Police, Anti-Fascism etc) as extreme.
That's not how the Overton Window works. The Overton Window says what kind of opinions are socially acceptable. If you describe those things are mainstream, they're by definition inside the Overton Window. Also, correct me if I'm wrong, but I think that you're more likely to be fired for being a nationalist rather than a communist, so that's another indicator that the Overton Window is actually on the left.
I see no evidence that that is the case.