[RickS]

Your measure of polarization is talking about distance on a political spectrum. This is perhaps the correct way, since it's literally polar.

The vernacular "polarization" here would be better thought of as a measure not of conceptual distance, but of the intensity with which that distance is perceived.

The left and right are both very centrist (with some wacky outlier issues), but they perceive the other side as being very wrong, which allows for polarization regardless of actual political distance.