ve2pid wrote:
But... returning to your document, I wonder why a balanced antenna
'see' that third wire, but the dummy load does not recognize that
third conductor. Is it because the common-mode current outside of the
shield has something to do with a kind of RF coupling?

Probably because the dummy load is a resistor not referenced
to ground. The antenna feedpoint impedance is a resistance
referenced to ground, i.e. not a resistor. The "third wire"
is also referenced to ground. You might consider it as a
kind of RF ground loop.
--
73, Cecil http://www.w5dxp.com