Political thought is a high-dimensional space. Many of those are correlated along an axis that people call right/left. When someone says that right/left is imaginary, what they are often really saying is that they prioritize dimensions in that space that are not strongly correlated with the right/left axis.

Tribalism being what it is, people can mistake the forest for the trees and think that the convenient shorthand we have for a correlated set of values is more strongly defined (more “real”) than it actually is.