He means the bottom of the stub is the feedpoint, like this:

+----------------------------- ------------------------------ -
|
FP
|
+-----------------

And that looks like an unbalanced feedpoint so it should result
in common-mode currents, sorta like an off center-fed dipole.

Not quite. Look closer - http://www.arrowantennas.com/j-pole.html
You assume it should result in common-mode currents.
And you know what happens when you Ass u me.
73 Al Lowe N0IMW