And? Eevee was with Fletchinder in the last XY ending, yet look how that ended up. The poster by no means absolutely, 100% confirms which is happening first (and if the poster were an indication of when they're happening, one would think that both would have been spoiled if they were to be happening this close together).
I really don't know why people use this as an excuse.
While I don't know one way or the other about the poster.
It's pretty obvious why Eevee was in the ending while Talonflame wasn't. And for a couple of reasons.
1. Talonflame is bigger, the ending was about dancing, it would've been awkward to have them animate Talonflame the same way Fletchinder was, it would be out of place. I'm sure its easier to animate a small object in the same fashion.
2. This was a new Pokemon for Serena, Serena has SOME favoritism on the production team so of course they were going to spoil a Pokemon on her team, months in advance. But let's not forget at the time we didn't know who was going to get who. At a time we thought Ash was going to get Eevee. If they didn't want to spoil Eevee, then there wouldn't have been anything out of place. They either couldn't work with Talonflame, or didn't want to spoil Fletchling's evolution.
Remember the ending did come out before the opening with Noibat and Eevee. Remember Talonflame wasn't on there, and remember that the Eevee's scenes only implied "it is going to happen" and Serena was going to catch it? It was a massive spoiler for a long off future.
They just wanted to spoil Eevee more so than Talonflame.
Now for the poster: Is it not possible that either it is been true, or it's been used for the similar purposes. Either way it works against you or works with you.
1. If its true then Noibat is evolving here.
2. If it's working the same way as the ending/opening then its obvious that they wanted to spoil Noivern over Sylveon.
The only problem that I have with this, is the upcoming episodes preview. They had no problem spoiling Eevee's evolution, but no the only thing we saw in regards to Ash was more Ash Greninja stuff, so if Noibat really was going to evolve in some off future, I can't imagine why Sylveon wasn't on the poster while Noivern was.
It makes sense to me why Noibat evolving here and wasn't shown in the preview, people just don't care to accept that reason as possible.
What doesn't make sense, is if they were trying to keep Eevee's evolution a secret, even though we were blasted by obvious hints all over the place, they'd show us that scene of that evolution ribbon in the preview. No other eeveelution has a ribbon, so unless the person who decided that was appropriate thinks his/her viewers are complete morons, it doesn't make sense for WHY then Sylveon wasn't on the poster. Well I suppose there's the whole they didn't want to reveal it at the time. But why would they want to reveal Noivern if Sylveon happens long before it? I mean we were fine with just Ash Greninja, I mean that was a spoiler of its own. So then why Noivern and not Noibat, were they that worried about how fans would perceive the situation since they don't seem to care about giving him a final Pokemon, so Noivern was to make it up?
I just can't think of any reason that makes sense for why Noivern is on the poster while Sylveon is not. I mean I suppose the fact that Serena hadn't caught Eevee YET when the poster came out, may have been an issue. How do you rationalize Sylveon when Eevee hasn't been caught, surely they wanted audiences to guess Eevee's evolution for a couple of weeks. I guess that's why.
But even then "Noivern?" Noibat wouldn't have worked just fine?