Everyone here is a game developer.
We have no idea what they did. Nintendo is stretched VERY thin trying to get games out for the U since they are practically the only ones still left making games for it. They need help to get things done. Namco could have done anything, who the hell knows. Could have some some track modeling, or character modeling, or animation stuff, or programming stuff, or lighting stuff, or texturing stuff.... or online stuff.
We know nothing.
Agreed.
No one claimed to know, though. Just a guess based off Namcos involvement with helping Nintendo with online in the past. Mario Kart 8's online infrastructure has grown, plus MKTV. No one has to be a game developer to make that guess.
Just like if Retro was in the credits, we'd guess that they helped with developing tracks. Doesnt mean that'd be true, though.