The word only makes an explicitly absolute claim. Pediatricians have read the same studies you and I have read, so they know the data isn't absolute. Therefore, they are claiming an absolute where they know there isn't one. You are right that I can't prove why they do this, but I think I can safely say most people persuade using untrue absolutes in order to realize an ideological goal or to make money, and the AAP is a non-profit organization. The quote is not something that should carry weight in a room of scientists or parents. I don't think I am over-thinking something that you've posted, especially now that you've asked me not to think too much about it.