Fort I'm sorry to inform you... but you CAN have games not show up on Steam search, but still be on Steam. It's a feature avaliable to developers to test Steam integration in the game BEFORE public release.
A lot of developers use this feature for Closed Betas.
You seem to be under the impression that Valve is in control of when a game goes on sale and when it shows up in search. However, the truth is that the developer decides and has control over all of that. The developer of Aven Colony can make it appear on Steam anytime he wants and he can give keys to it before it's publically known. JUST LIKE WITH ITCHIO, THE PLATFORM DOESN'T HAVE INVOLVEMENT WITH THE PRODUCTS.
And what I find even more strange about your argument... is that you have a Steam Key from the developer, but you won't activate it because you don't believe them? What do you THINK that the key will do if not activate Aven Colony? It would either work or not work... if you don't want to try it, PM it to me and I'll see if it's Aven Colony lol