I have some minor background in control theory. After going trough the paper I had a very same thought thus I googled and found this: http://www.control.lth.se/media/Staff/KarlJohanAstrom/Lectur...
After checking out the slides I kind of feel that there might be some oversimplification in the used simulator, like producing _the desired lean angle_ might not be enough. But I am no expert.