I talk about my startup ideas all the time... in person. I haven't posted them online. The reason is that the former benefits me, and the latter doesn't. The feedback I might get from, say, a hacker news comment just isn't as good as what I hear from people I talk to.

So there really is a pretty big difference in what is meant by "stealth mode". Putting up a landing page to gather emails to hear more when you're ready, and only talking in depth about the product in person to help make it better, isn't so much as stealth mode as heads-down-mode.

Not telling your friends because of fear of your idea being stolen is really quite dumb. Not just because they wont steal your idea, but more importantly they might make your product better!