Personally, I don't see the satire here. Satire is meant to mock something as a way of trying to correct it. But I do not think that Twain is mocking anything here.
In this scene, we are seeing Huck's character develop. We are seeing that he cares enough about people in general to want to save them, even if they are bad people. There is certainly irony here -- he is doing something bad (lying) in order to try to achieve something good (rescue the men). But irony is not the same thing as satire. I don't think that Twain is using this scene to criticize any part of human nature or anything about the society of the time.
What do you think?