The Libertarian Position on Unconquerable Drive

In the book The Science of Freedom, p. 87, two exceptions are cited in which even the libertarian admits that there is no free choice. The first is 'an impulse that cannot be conquered.'
Does this mean that in such a case there is no free choice at all?
If this is the intention, why would a case in which the "mental topographic mountain is too steep" (as described later in the example of the saddle) be fundamentally different from a case in which the mountain is less steep? Or is the difference merely quantitative, but rather there is a certain level of 'steepness of the mountain' that a person cannot be required to overcome and therefore should not be punished for such acts?