Correct but usually there is a motive or something to gain from doing so for people who are inclined to try and spread the lies. I don't see any reason for that here, if nothing there is immensely more to lose from pulling those sort of shenanigans. So taken in context I believe the screenshots to be authentic. So in this instance I would ask again what you would need to see for proof?
I'm always interested to find more ways to authenticate online conversations, good tools to have with all the he said she said stuff that goes on.