You will not see a condition of no switching in FPWM mode if you are operating the MAX16956 in normal operating conditions. The no switching condition can be thought of as a test mode. When we are testing the MAX16956, we can put the part into a condition where the FETS are not switching. This gives us a measurement of the quiescent current of the part.
This parameter is given because it helps with efficiency measurements. Generally, the efficiency of a Buck converter tends to drop off when the output current is less than or equal to the quiescent current of the part. This parameter can be helpful to determine the lowest load current you would want to pull from the buck converter.