Good point. But Petunia's knowledge of dementors and Askaban was obviously intentional in OOTP. How did she learn that. I can't imagine her asking Lily.
In an interview, Rowling says that Petunia overheard a conversation, that is how she knew. That's all she will say but she says it ties in with more stuff in the seventh book and will become important. A squib has magical parents. I don't think Petunia can be becasue her parents were none magical.