Saying that you have no legal obligation - i.e. that you have no obligation to help someone - i one thing. Saying that it’s ‘their problem’ - and implying that it is not also your problem - is a whole different matter. In a very real sense; it is also your problem, if not purely because it harms the intellectual division of labour that would be possible if they weren’t coerced to death. You might say: ‘well, I don’t think that’s a big problem, so I won’t act upon it and you can’t legitimately force me’, which is true. But that’s not the same as implying that it’s not your problem.
I agree with your other 3 points, but I think this qualifier is important. Just because ‘we’ libertarians think that the use of force ought not be used to solve a certain problem, doesn’t mean that it couldn’t be a problem.