This is completely unfounded advice, and in my opinion, stupid advice. Ideas do get stolen, regardless of what you've read.
I believe that it may happen, but it feels like an urban legend - I've never actually seen it or heard it.
The writer would have been closer to the truth if he said that founders generally err on the side of secrecy.
And if you don't know anyone this has happened to, how do you know that it actually happens?
A potential sale situation is different from taking the first steps on the idea. You had already started building the company.
I think managing the disclosure of sensitive information is a totally different topic (one which I wish I knew a little more about).