Seeing success in the family certainly helps. You are right that not every woman (or man) has had that opportunity; however what Sandberg tries to do is to exactly help them by providing an example that they were lacking when growing up.

I'd say more from my (very anecdotal) experience: I'm in a male-dominated and technical field, but among (very competent) women that we do have, it seems that growing up in a family where mother is decidedly more successful than the father is quite a common trait. Good role models to follow -- and not just abstractly good, but examples that suggest that women can compete as equals and win on merits in real life -- do matter for confidence, and confidence matters for everything else.