As far as I'm concerned, sure Sylveons type is purposefully hyped (so I guess u can argue that they're trying to surprise us) but We have to remember that this is the first movie dedicated to the eeveelutions. They could just be hyping it for advertising purpose.
In fact the entire eeveelution line has been promoted a butt load of times at the pokemon centers too.
So I don't think there real intention is to conceal Sylveons type more so they want to put focus on the Eeveelutions as much as possible. Especially since the Eevee short will be featured in theaters in Japan.