How is that rude? They are just stating the fact of how liberal is used colloquially, you don’t need to jump on the offense. Their username is literally “naive”.
In political theory liberalism is considered right-wing because it is a pro-capitalist ideology. “Liberal” is used to describe anyone somewhat progress in the US and among some others but the term is used differently by political theorists and most people outside of the US. Many academics are right-wing, most academic institutions are explicitly right wing because they are pro-capitalist even if they are more “progressive” socially. Though the students and faculty are typically much more left than the school administration.
The meaning of that phrase about “liberal bias” is reflected in things like the rich exploiting the poor and polluting the earth, “liberal” viewpoint but true in reality. Or the rich manipulating elections and using the military to further imperialism, leftist “opinions” but verifiable in reality.