Somebody with more funds and connections could ship your idea before you rendering yours obsolete. In the case of a hardware based startup they could even patent the idea first.

Anyone with the resources and gumption to outperform you at your own idea is someone who has their own ideas. They aren't looking for ideas to steal, because ideas frankly aren't that hard. They're looking for progress on the ideas they have.

The whole point of the "tell everyone about your idea" mentality is that your idea isn't important/special enough to be copied.

It's supposed to be humbling advice to directly contradict this argument.