The meaning is that when things that should not be political questions because they have objectively correct answers do become political in recent years most of the time it is liberals whose positions match the objectively correct answer.
Note that this doesn't necessarily mean that liberals are more often correct than conservatives on how to deal with those things--that often is something that does not have an objectively correct answer and so is something that people can reasonably disagree over and so can reasonably become political.
For example consider climate change. How to address climate change is something that does not have an objectively correct answer and so you can't say that any given political group is right or wrong on that.
However, the question of whether or not the increases in the levels of CO2 in the atmosphere since pre-industrial time are most due to human activity is a question that does have an objectively correct answer. The C in CO2 comes in several different isotopes, and by looking at changes in the ratios between those isotopes in the C in atmospheric CO2 it is possible to determine that most of the increase has come from burning fossil fuels.
If a political group is taking the position that the rise in CO2 is not due to human activity they are objectively wrong, and the phrase that reality has a well known bias against that group is a way of highlighting that.
The meaning is that when things that should not be political questions because they have objectively correct answers do become political in recent years most of the time it is liberals whose positions match the objectively correct answer.
Note that this doesn't necessarily mean that liberals are more often correct than conservatives on how to deal with those things--that often is something that does not have an objectively correct answer and so is something that people can reasonably disagree over and so can reasonably become political.
For example consider climate change. How to address climate change is something that does not have an objectively correct answer and so you can't say that any given political group is right or wrong on that.
However, the question of whether or not the increases in the levels of CO2 in the atmosphere since pre-industrial time are most due to human activity is a question that does have an objectively correct answer. The C in CO2 comes in several different isotopes, and by looking at changes in the ratios between those isotopes in the C in atmospheric CO2 it is possible to determine that most of the increase has come from burning fossil fuels.
If a political group is taking the position that the rise in CO2 is not due to human activity they are objectively wrong, and the phrase that reality has a well known bias against that group is a way of highlighting that.