Both Infernape and Torterra reached there final forms around the same time. If the picture I mentioned is legit it's a bit odd that Litten would be fully evolved and Rowlet isn't even a Dartix
But there are issues with it.
First off its not really acting like Ash's Incineroar, more like a random one.
There's also Popplio and Rowlet, which doesn't make sense to me, since I very much doubt they'd ONLY evolve Litten.
It's extremely WAY TOO early to even suggest Ash having an Incineror, I don't expect it to happen UNTIL at least early 2018 and I doubt that image would exist at this moment. AT MOST maybe Torracat by April or May though that's pushing it..
Finally it makes FAR more sense for ANOTHER Incineroar to show up, simply because I very much DOUBT that Ash will debut Incineroar, and the Incineroar art looks exactly like that a debut, and otherwise I would hate for Incineroar to show up like 5 episodes before Ash's.
Now you're probably asking....where's Litten? Well it probably could be there and not drawn or maybe not shown.
Let me also put it in a different way: Ash's Charmander the earliest a starter EVER evolved (for Ash) "fully or just once" was 32 for Charmander to Charmeleon, and 3 for Charmeleon into Charizard for a total of 35 episodes.
Even if they were duplicating Charmander's level of quick growth, I doubt they'd have concept art of it in March (when the concept art was released), since we should be getting THAT particular episode with Incineroar, if its concept art sometime in February or March, and that's not even 30 episodes either.
OTHERWISE, Litten should still take at least a YEAR before evolving into Torracat, and ESPECIALLY then I doubt they'd have concept art of it, IN March of 2016.
Sure, maybe they can have PLANNED concept art, of planning with Ash with an Incineroar, and that might BE that concept art, but I'm most certain that things HAVE to change while they are making the series, especially when they probably have to gauge on what Pokemon, consumers like. And if a majority of people want Ash with a Decidueye over Incineroar, they probably have to give him a Decidueye, granted they gave 5th gen, a giant middle finger, since I very much doubt Pignite was the best starter, but I could be wrong about popularity.
So the concept art of Incineroar, should be taken with an extreme grain of salt. Unless you really think Litten is somehow going to pull an even worse Charmander comparable to Goomy (without the release)