Maybe because the trainer is seen wearing a football uniform?
If he was still wearing his normal clothes, or other kind of costume that hinted at "sports" in general or some kind of new fancy thingy like the one for Poké Rides in Alola, everyone would've assumed it was just a normal stadium like the ones from the anime. But why would the trainer have to change his clothes to a soccer uniform out of all things? Or why don't we see him outside the stadium wearing that outfit?
I mean, after all, it's just a costume, and it doesn't confirm anything, it is even most likely going to be optional, but with the precedent of Trials instead of gyms in Alola, I don't find that crazy that most of us thought about a new soccer-inspired mechanic replacing the gyms after watching the trailer (which I'm totally not excited about if it's the case).