Output impedance of a 4mA to 20mA current loop
Look at the circuit in the data sheet: -



The output loop comprises: -


The chip itself and, as a current generator will have a very high impedance
A series voltage supply (VLLOP) that powers the chip
A series load resistor (RL) that is used to measure the loop current


So, from the perspective of the power supply VLOOP it will see a fairly high impedance due to Q1 being in series with RL plus a little bit of current that passes into the voltage regulator on the chip.

But from the perspective of looking at the loop across pins 7 and 4 you will see VLOOP in series with RL and, as a dynamic impedance will equal RL because VLOOP will be a voltage source with a very low output impedance that is  &lt;&lt; RL.