Considering each of them have three Pokémon maybe again it's like a Battle Frontier feature i.e they're set at Level 100 specialising in their typing which would be accessible post-storyline.
Also, although there are no Johto/Hoenn leaders there currently that doesn't rule them out..
"You can download more trainers via special distribution methods such as the Nintendo WiFi Connection, as time goes on. These include many of the special trainers seen below"
Even then, we don't have full details on it yet.