It's one of those public secrets. Just about everyone knows, but the company refuses to officially admit anything until THEY choose to. After all, that's their luxury. If they don't want the world to know that they officially acknowledge Deoxys, they can do so. After all, telling everyone everything ruins some of the power they hold over the buyers. If they told everyone about FRLG the second it started development in Japan, people here would be sitting and waiting and waiting and waiting, and eventually lose interest. As it is, some people don't yet know, and thus will be surprised when the info is released, and the surprise alone may convince them to buy the game.
Especially saying that the game contains secrets not previously available in the original games. That'll hook buyers like crazy, without releasing any major details.