What does a TC have to do with ISO? Not a thing.
We are talking that you shoot at the limit of what you can.
So imagine you have a 600mm f4 and you have to choose a camera for a far away subject:
- D500 + 600mm f4: gives a 900mm FOV @ f4
- D5 + TC2 + 600mm f4: gives a 840mm @ f5.6
(you might work with something further even)
I would probably shoot at 1/500, in a limit situation (not moving subject).
To have the same exposure I would need one stop extra on the D5 in ISO (e.g. D500 @ 6400; D5 @ 12800 ISO), which would give similar results in noise as the D500 according to the person making the argument.
The difference could be Dynamic Range, the kind of noise, etc... the person is making the argument that the issues introduced by the TC are pushing the Image Quality advantage to the D500.