2-port shunt-thru: why is GND isolator unnecessary at HF?

In lesson 7, it's stated that a GND isolator (transformer or solid state) isn't needed for HF measurements. Although I use a J2102B, I'd like to understand why.

I've wondered about this since reading that coax pigtails used in a VDD-VSS measurement carry a good differential signal at HF, and it's not necessary to connect separate VDD and VSS cables to 2 scope channels for subtraction.

My guess is that at sufficiently high frequency, the distributed L of the cable presents a sufficiently high reactance to create a Z0~=50ohms, which dominates the distributed R of the cable shield. That makes the drop across the R insignificant, thus attenuating the CM signal.

Is that right?