Interesting question. Personnally, I think I would never be able to train my forearms in this manner to failure.
By "sets" they mean rolling up the rope completely is one set, I'm pretty sure.
But for me the pain would be simply too much... I don't think I can ever get to failure with high reps on the forearms for that very reason... Can anyone? They'd have to be very deep into masochistic behaviour if you ask me
Anyways, you can train forearms to failure with wrist curls/reverse curls, if you don't rep tooooo high, as in 8-12 is doable. Try failure at 25 reps and you will notice that it is the PAIN that makes you stop, not anything else.
Train them, train them hard, and they will grow. Forearms are so difficult to overtrain that for all practical purposes you can treat them like they will never be. Forearm size is more often than not an indicator of how LONG a bodybuilder has been training, for that reason.