I dont, I'm just using common sense. The platform has been announced already. What would prevent Psyonix to say "We're making Rocket League for Switch" ?
Unless they have a specific marketing plan, publishers wont hold announcements of ports that could've happened for Switch event.
1) The project may fall apart at any stage. Knowledge of this can harm the reputation of both parties when seeking additional ventures with each other or other parties, and the reputation amongst consumers.
2) Knowing that a platform holder / publisher is in discussions with a particular party gives other (potentially currently unassociated) parties negotiating leverage. This can relate to revenue splits, marketing space on digital storefronts or even things such as dedicated support from platform holders.
3) (Specific to this example) Announcing that RL is in development for Switch, without indicating the full extent of multiplayer-cross compatibility can be damaging for both the developer and for the platform holder. In the instance that cross-play was
allowed by Nintendo, this could potentially lead to other developers also wanting access to this feature. This in turn could lead to additional delays of products that were already forecast a release window
4) Any promotional or exclusive in-game content will have to be "contracted to hell" by both parties to ensure they aren't getting shafted by the other one.
Making games is a
business. In many cases it can be legally frustrating and complex. It is seldom as simple as saying "I'm gonna make this game for X platform" and then the game appearing a certain number of months later.