Kinda like what others had said already, I personally think the Alola Leaders are like the Gym Leaders, especially if you add on the ?captain to each "leader", it'll make up 8 people you battle, right? (Just like the 8 gyms, 8 figures you battle)
It's just unconventional, more quest-like instead of the traditional gym-building.
But them being the Elite Four? No. I don't think so. They wouldn't release the "Elite Four" this early on and the figures you battle that they'd released seem more casual than an Elite Four member. (I hope this all makes sense lmao.)
Maybe the "Elite Four" will be like another trial after you're done completing these trials.
Again. Hope this all makes sense.