How can I model/understand this isolation module's CMR rejection ability?
A truly floating input has infinite CMRR by definition, but no such input actually exists.... 

In particular there is always capacitive coupling across the isolation barrier and capacitance to ground (Including from the cable) to screw things up. 

Now it is true that the higher you can make the common mode impedance at the receiver, the less minor inbalances in the cables and source matter, hence the existence of boostrapping schemes like the one THAT Corp use for their audio line receivers.