I think the explanation for the strong/weak-willed is pretty straightforward. INFPs put "values on things". Some things they value very highly, so highly that it seems part of them and when these things are threatened, they never back down even when they are in the minority view.
On the other hand, there are the majority of things that the INFP barely cares about and can just roll with things. That makes them very good confidants because they can see the other person's point of view is as valid as their own. But the drawback is that it makes INFPs seem meek and soft because they tend not to push their own feelings, and would defer to the preference of the other party in these situations.