My guess is they chose him specifically to have a leader ability that relates to the Civ V Polynesians while still representing a single culture rather than a mishmash. His moko is unmistakably Maori, as is his outfit which still retains the feeling of an explorer. Above all, in the cinematic his necklace is of te wheke, the octopus - Kupe defeated a giant octopus on his journey.
Yeah, the Māori would've used similar ships to migrate to New Zealand. They're very important in our cultural traditions, and most tribes will affiliate to one or more of these "waka", as we call them (or "waka hourua" when specifically referring to the double-hulled, oceangoing canoes). These tribal waka usually have names. According to Māori tradition, Kupe himself travelled to New Zealand aboard a waka called the Matahourua. It could've been that very waka that the trailer was depicting.
I agree, at this point I think the Maori are definitely confirmed and won't be a blob Polynesia.
Due to Kupe being the leader I think he might be the leader that will get his own UU, the waka shown in the trailer which could cross oceans sooner, while the Maori Warrior will be the civ UU.