To illustrate my point; there was no need for either of the mentioned successful startups to keep their "idea" a secret until launch, in fear of someone "stealing" it.
What I mean is that had someone heard of their "idea" before launch, it would have changed nothing. It's all about the execution.
Conversely, many would-be startup founders I've talked to seem to think that their idea is the most valuable thing they have. That they need to keep their super-unique idea under wraps, for fear of it getting out and someone else doing it instead.