When it says MUX Off, it does not mean VCC=0V. It means that the MUX switch is open. VCC should be in the normal input range for this spec to apply. Any voltage change on the MUX input will be reduced by 120dB on the MUX output if the input voltage frequency is less than or equal to 1.8MHz.
IDC_LEAK(IN) (10nA max) is a better indicator of input impedance of the MUX inputs. This spec also only applies when VCC is in it's normal operating range.