Without seeing the design in question, I think this still demonstrates the ambiguity the post is discussing. Is the icon what will happen, or what is currently happening? Is it the state, or is it the action? Maybe this particular design was unambiguous, but the description of it isn't.
If the light is beside the switch (rather than being the switch) then I'd say it clearly and unambiguously indicates the current state of whatever the switch operates?
(A light also has the advantage of being a skeuomorph - much as those are now out of fashion - we all know how to interpret indicator lights in the real world.)
The standby light on a TV in on for standby and off when viewing because a) it could be distracting and annoying for the extra LED to be shining while viewing in a darkened room, and b) the "on" state should be pretty obvious because of the stuff on the screen.
In electronic music instruments (which derived interfaces from electronics lab equipment) it's quite intuitive there's a switch with a light on next to it the next state is off and the light switches off.
