measuring output impedance of op-amp
They are trying to show that the feedback mechanism compensates for any resistance on the output of the opamp driver stage (hence the term "perversely").  The opamp compensates for the new 1K ohm resistance by increasing it's drive so that the new feedback point again matches the positive input pin.  You have to think of the (non-load) 1K resistor as having been internalized into the op-amp in this setup and thus compensated for the "horrible" 1K output impedance.

Bit of misnomer, I think they're measuring DC or a sine wave with no phase detection (depending on the frequency there may be visually notable change in the phase shift, indicating that the impedance actually has changed).