Input current and quiescent current are two different things. Input current refers to the current drawn by the IC when the part is operating in a loaded condition (that is when there is a load at the output). The input current will then depend upon the current that the load is drawing from the part. For example the input current in this case can be calculated as follows with relative accuracy:
Vin * Iin * Estimated_Efficiency = Vout * Iout
Estimating an efficiency of 90% ( typical for this part ):
2.2 * Iin * 0.9 = 5 * 20
Hence Iin comes out to be 50mA when the load is drawing a current of 20mA.
The current consumed by a circuit when it in a quiescent state is called a quiescent current where the quiescent state is defined as a quiet state in which the circuit is driving no load and its inputs are not cycling.
Hence 300nA is the typical value of quiescent current and may change depending upon how the part is configured. The appropriate value of the quiscient current can be calculated by following the instructions in the datasheet.