The analysis seems to disagree with the data shown in the graphs about the labels of "left" and "right". The author seems to be confusing left/right? It's not just the first stacked bar, but also the third (avg %bias)
I think what you’re seeing is a confusion coming from ground news own terms. They call something a “left wing blind spot” when it’s a story that does not appear in left-wing sources, meaning that it only appears in right wing sources. The blog copies that terminology here, but it can lead to some confusion.