I think you've fallen for the fallacy associated with "Ideas are worthless".
A lot of people assume that means "share it with everyone" - which is definitely not what this article is suggesting.
Share it with people you know, people you respect, people who you think will give input - but share it personally
If you are going to attempt to execute an idea throwing it onto HN (for example) does potentially devalue you it for you. The amount of extra input is potentially useful (though I doubt by much) but the chance of it being implemented by another shoots right up too :)
(of course; if you dont plan to execute now, or dont see yourself executing then sharing publicly is a good idea)