I believe this is not true. I believe that the "overton window" [0] has shifted right, far more than the left walked out of the room: the window of discussion leaned back into racism, anti-unionism, anti-abortion, selfish, anti-tax rhetoric.
The L has not moved very much, the level of street complaint, and asserts to the need to defend ground are not "moving futher left" they are a digging in, in-place.
Due respect to the writer, I think they're wrong.