wtf. Dictionary.com says "Objective most commonly means not influenced by an individual’s personal viewpoint—unbiased (or at least attempting to be unbiased). It’s often used to describe things like observations, decisions, or reports that are based on an unbiased analysis."
I'm not sure why you quoted it, but "Reality has a well-known liberal bias" is a joke you make at the expense of right wing people for not believing in reality.
What they mean is that if you approach some issues with "[no influence] by [your] individual personal viewpoint" you end up running into a leftist or slightly moderate viewpoint.
For example, take climate change. If you come at it looking only at the facts, you'll recognize we need more renewable energy and climate change poses a threat. Donald Trump, to contrast, in intending to put more money on oil and gas and remove subsidies for renewable energy.
Or, if you prefer, the economy. It's more or less undisputed that tariffs will hurt the GDP and overall economy of the US. However, Donald Trump claims tariffs will help the US economy.
Or, perhaps what the GOP has treasured most of all these past few years, the culture war. For example, gender-neutral bathrooms. From a neutral perspective, forcing trans people to use the bathroom of their assigned gender at birth will backfire tremendously. Instead of having trans women in women's restrooms, now you will have big burly and hairy trans men. Or look at gender affirming care, we have statistics about gender affirming care lowering the risk of suicide. But the right claims gender affirming care causes suicide and has a high regret rate.
Those are just a few examples, but if you look at popular conservative policies and then try to reason about them you kind of hit a wall.
By the way, not knowing the history of the reality thing I looked it up - it came from a Colbert joke about W Bush's popularity https://youtu.be/UwLjK9LFpeo
https://www.dictionary.com/e/subjective-vs-objective/
I'm not sure why you quoted it, but "Reality has a well-known liberal bias" is a joke you make at the expense of right wing people for not believing in reality.