Short in wiring?

[Chamberpains]

If your using a light tester for a 12v circuit, when you place the tester where you described then of course it'll light up. You made a simple circuit. Pos. post then the load then the neg.(ground). Your's is like this, 12v source(pos. post) connected to the little itty bitty light in the tester connected to the huge long battery cable that ends on the starter post(which is now acting as a ground for the itty bitty tester light). There is more than enough resistance in that big and that long of a cable to act as a ground for a little light bulb in a light tester.

With the ground post you have no 12v (pos) source. Its all grounds on both sides of the tester. Nothing there to light it up.

You may want to share the youtube video link so we can see what exactly they were testing and how.
Youtube is a great place to troll the ever living stupid out of society (tide pod challenge) and also home of the dumbest techs screaming they know what they're talking about when they very clearly don't.