Earlier quoted context omitted.
There's two ways to lean left. Do some leftward leaning, or stand still while your reference point ("the right") leans away. I personally think both of these have happened. I bristle at the inference that only one of them is taking place.
Except evidence has shown the the left has moved further left than the right has moved right. Both have moved but the left has moved further left in the US.
The article is a few years old, but by most measures, the Democratic party has been moving from the center-right toward the center-left, making it a relatively centrist party on the global stage, while the Republican party has remained firmly right-wing.
Your comment appears to be true, but what we're seeing may simply be America rebalancing herself for the first time since the lead-up to the Cold War.