Honestly, I think every parent-to-be (first-timer) is delusional!
I know I was. I wasn't a complete twit about it like the people you're describing, but I laugh now at how clueless we were and how much we thought we knew.
The best part is that the laughs keep coming. I thought because I eventually knew what I was doing with a little baby, that automatically meant I'd have a clue how to deal with that same child when he became toddler. Well, we're entering the toddler phase now and I am once again realising I have no idea.