I think they mentioned it because at the time, there was no guarantee that the Switch would be a success. If the Switch ended up being another Wii U, Nintendo would've been in trouble. So Nintendo decided to prematurely announce FE to get people to buy the Switch. I think the same reasoning applies to the timing of the announcements of Metroid Prime 4 and Pokémon Switch. We don't know anything about those games other than that they exist. Now that the Switch is a success, Nintendo probably feels less pressure to announce more news (although if the Feb. direct rumor is true, FE may be talked about there).