The other commenter is correct that Colbert coined the term. It's true that there was a lot of talk in that era about who did or didn't adhere to "reality" and how that related to partisanship, and a segment of the blogosphere that described themselves as the "reality based community," in reponse to that Rove quote, but none of that background context has anything to do with anything when it comes to the fact that the phrase itself, "reality has a liberal bias" is something that came from Colbert.

Of course it was in an intellectual climate where those words were charged with political meaning that made them salient enough to set up the joke. But to say it didn't original with Colbert would be to say that phrase itself was coined by a different person, and if it was, you could say who it was, and when and where. Otherwise it's just a confused detour into the weeds that lost track of the original question of attribution.

This is a huge detour, so to tie it back to the original thread, I think talks of "bias" are poisoned by a false assumption sides are equally estranged from scientific truth. It feels fair to make a both-sides argument, but it really just doesn't hold water under any serious examination especially when it comes to which party affiliation more systematically aligns with, responds to, and believes in scientific consensus.