I think Progressive Realist is talking about the fact that politics (and most things anyway) are subjective and that they lack an objective truth (since it's all a matter of perception).
This is good! Leads the way to interesting stuff.
This seems like it's true, but quickly dovetails into meta-ethics. What do we mean when we say "good" - an extremely hard question in which differing answers create completely different moral worlds. And the claim "there is no 'bad' side" is a very modern reaction to a Platonism that supposes the necessity of a good "substance".
Most modern research in ethics focus on the creation of value through judgement, calculation and rule-making. I'm thinking of Moore here (though I never read him, so don't quote me on anything!) The idea is that moral principles are distinct from the way of the world, so-called "naturalism". In this view, there is a good and a bad, but they have no relation to any non-moral judgement.
To apply that above thinking to this passage: It can be true that politics is irrational and highly subjective. That has no bearing on whether a certain policy is right - and certainly no reason for us to stop arguing about its rightness.
But now we remember that politics is not purely moral. A politician can have epistemic - concerning knowledge - duties to his constituents; transparency, accountability, frequent communication, whatever. To be more cynical, a politician is elected to represent interests. Even in this case we can judge whether that politician is representing those interests accurately.
How do we respond to a side that "creates their own truth"? The easy way may be to argue based on moral principle, but that is sketchy - as I discussed above. And, since politics has both moral and material dimensions, we can also argue from this different material angle. In this case the politician is "bad" for a betrayal of his role. Notice that this is now a two-step procedure. First we argue from observation, then connect that with morality.
To connect this passage with what I've said, there is a moral intuition against an "inconsistent role" played by the politician. Here's a possible origin for this; we elect politicians based on their campaign behaviour, but cannot control how they act afterwards. Institutions can be designed to keep them to account (this is the heart of the principal-agent problem). But how can we keep them to account when we're not the ones designing them? The only way is to establish a morality against inconsistency to guarantee vigilance against those elected.
I realize I've complicated the discussion enormously, but this is great; we're not really talking about cognitive bias but political philosophy. Psychology bores me to death; philosophy the very opposite.
