I understand the Gleipnir is supposed to be easy to undo, whereas this is quite difficult to undo.
Yes, that was a surprise for me, too. I guess it has something to do with the fact that, in this "simple hitch a la Gleipnir", the nipping loop is in touch with / is squeezed on the pole s immovable, hard surface. Now it is not only a nipping loop, as in the case of the Gleinir binding knot, but a kind of riding turn as well. So, its gripping power is greater.
The same happens also when we use this mechanism to keep the two ends of the Well Pipe hitch (1) under tension. I would say that, in this later case, the result is even more impressive, because, when the two ends of the Well Pipe hitch are kept under the tension the "simple hitch" can deliver, the effectiveness is "propagated" through out the multiple coils, so that, in a sense, it is
multiplied !
1)
http://igkt.net/sm/index.php?topic=2075.msg16893#msg16893