Biased relative to what? The racial makeup of current employees, of local/national demographics, industry average demographics, or demographics of low-performing employees?

Say the fired employees are disproportionately (compared to company demographics) of race X. Responding to accusations of bias, HR defends this with "Employees of race X at our company happened to have more low-performers than average".

Let's close our eyes and imagine the fallout for different values of X.