It's not game art, it's production sheets from the anime. It was 100% confirmed real when Mallow was revealed. Also if that wasn't enough, the TCG leaked out the final evolutions and their names before they were officially announced anyways.
The scene with Ash/Mallow sitting around an Incineroar never happened in the anime though. Assuming that's Ash's Incineroar, I find it hard to believe they have model sheets way back in 2016 for something that wouldn't happen for another 2 years into late 2018. Ash still has Torracat, and while I know anime episodes are planned out almost a year in advance, I doubt it's 2 years in advance.