I’m still listening to that audio book the story of which is set in Tennessee and I’ve noticed that the characters say ‘I know it’ as a confirmation. Here is an example. Frank: ‘Shelby is pregnant again, can you imagine?’ Mike: ‘I know it’. Why not ‘I know’?
To me, “I know it” has more of a sense of agreement to it - sort of a combined “I know, you’re right”. “I know” would mean I too heard that she is pregnant, while “I know it” means not only do I know this information but I agree with you that it is not a desirable situation.