First of all, if the tour guide had hung from the outstretched arm and done chin ups that would be head turning. As it is, you have no idea how much force was applied by either person.
I first experienced this test by a health food guy demonstrating how white sugar was bad for you back when I was a kid. He would tell you to stretch out your arm and then pull down on it without overcoming your resistance as show in your linked video. Then he would ask you to hold a small amount of white sugar and would then "easily" force your outstretched arm down supposedly demonstrating how white sugar weakens you.
The first thing I noticed when doing the test on someone else was that their resistance didn't change but in order to obtain the desired result and avoid embarrassing myself, I had to pull down noticeably harder. Of course, the test subject didn't realize I was applying more force because they thought they had been weakened.
A reasoned and plausible answer but one unsupported by empirical evidence. I would love to test this hypothesis.