Idk, maybe my opinion will change after the announcement and it will be obvious why it had to be kept secret.
"Hey, I've got this fantastic idea!"
"Really! Tell me all about it!"
"We're going to blah blah blah."
"Uhh... did you talk to a lawyer about that? That might be illegal."
"Uhh... really? no."
"And where will you get the money to do it?"
"Uhhh... haven't figured that out yet."
"And if I want to participate, what should I do?"
"Uhh... I dunno, we don't really have a process for that yet..."
I made the mistake of asking a reporter when her deadline was, because if it was far enough away I could tell her about the neat idea. I should have kept my mouth shut or sworn her to secrecy.
Announcing a half-baked idea is counterproductive; you'll just get a gazillion questions that you can't answer, or, even worse, people will assume they know the answers and then be disappointed when it turns out the idea they thought they heard you describe isn't the same idea they thought they heard.
Announcing a fully-baked idea and then modifying it based on what everybody thinks and how it works out in practice is the right way to do things, in my humble opinion.