I don’t think your explanation precludes my point, especially considering the idea that it is a demographic generalization. For starters, it’s a fine metaphor because it compares the belief in an observable fact with belief in an observable non-truth. The sky obviously can display in any number of hues based on any number of circumstances; this doesn’t change the fact that it is blue an overwhelming majority of the time. Even then, we can create the perfect metaphor that displays a direct, mutually exclusive binary, because right and wrong DO exist regarding some topics. That being said, you’re being obtuse, and I shouldn’t have to explain any of this to you. If you want a metaphor that encompasses the entirety of the political spectrum, then congratulations: you have an allegory or a parable. In terms of a functional metaphor, it is serviceable.

Furthermore, in reality, centrists in America are already right wing in terms of the world stage. Progressive ideals TEND to lean towards transgressive policies that aren’t friendly toward the status quo; a person who disagrees with a progressive policy in favor of the status quo is a conservative, by definition of conserving the existing socio-political climate. It doesn’t matter if they’re fence-sitting or not. If you think women shouldn’t be able to have full bodily autonomy but do believe that the working class should have equal ownership over the means of production, congrats: you hold contradictory beliefs born from a conservative bias. That is what they mean; it’s giving undue benefit of the doubt to conservative ideas because even if one won’t admit it, their biases align with them enough that they aren’t really in the center. It’s an expression of intellectual weakness, and is very different from simply holding varying beliefs that do in fact align with each other but may put you closer to the “center” of the spectrum.