Still its placement in the poster was quite prominent though, which's my point actually.
Ilima's a stylish dude. Who's to say that he didn't get his Eevee a new haircut? Quite possible.
same could be said about Mega Sceptile in the XY&Z poster whose position was quite prominent.
How do you know that Ilima won't show up frequently in the future? He could likely be one of Ash's rivals this saga, and thus could possibly show up more frequently in the future and could be one of Ash's major rivals in the Alola League.
Reminder that when the XY&Z poster came out, people said: 'Sawyer isn't even present in the poster', 'Sawyer is a novice who can barely battle, why should he get a Mega?', but.....oh well, things happened.
If you wanna take the bait go for it, but I'm not inclined to believe that it's indeed Ash's Eevee.