i dont think you need a super flexible back, in the picture that's posted in the inital post you can see her back is relatively straight and not very arched but you will find this much more difficult if you have a very SHORT torso! there's a bunch of tricks to getting into position– this is what i've found works for most people (this is assuming you climb with right foot behind and are coming to a sit with your body to the left of the pole):
go from a sit. grip the pole between your knees and calves and sit your butt a little bit back from the pole. if your crotch is right against the pole, you will get vey very stuck when you bring your side around. lean forward and try to lay your chest on your thighs. THEN twist to rotate your chest and point your left shoulder up the the ceiling. when you grab for the pole, bring your left arm OUT then up– as you are rotating your shoulders, you should open your knees so that your hips open also– if you watch this vid of me doing it at about 2:50, (https://www.studioveena.com/videos/view/4d232d0b-3570-41a0-bf14-32d00ac37250) , you can see my knees gap apart a second before i grab my foot- if i didn't spread my knees, and kept my hips facing forward, i wouldn't get as open as i need to in order to reach that right foot. the version in the pic is the same trick, just without grabbing the back foot. the pole is held in tension between the back of the left arm, your back, and the inside of your right thigh. hope that helps!