Just like stereotypes, bias has a generally negative connotation but it isn't actually just a proxy for "and is statistically inaccurate for the population". The misapplication of the population information comes into the age example used on page 2 - just because you'll score more correct answers if you guess the person in their 70s has memory issues compared to the person in their 20s because it's true of the population does not mean you actually have enough information to just say that's how it is for those 2 individuals as that example lays out.