Calculation of the slew rate of an inverter with active load
$$\text{Slew rate} = \frac {\text{current going out from the circuit}} {\text{capacitance at the output}}$$

The output current is the difference of the currents of the input transistor minus the I_bias. It is a function of the load capacitance. 

The formula you copied here is valid only for rising edges on the output. Since I_bias is smaller than the maximum current of the input transistor, this slew rate is worse than the falling one, and hence some might consider this value as the "slew rate" of the circuit.