Nintendo is a conservative company with strong, well known brands (Mario, Donkey Kong, Legend of Zelda, etc.). I'd be surprised if they rolled over and let this slide.
I'd be doubly surprised if they ever open sourced anything.
I've always viewed 4chan as hardcore chaotic neutral. They're quite happy to find novel solutions to mathematical problems, DDoS people they don't like, openly work with leaked source code, create a new chat system (tox), troll celebrities... they don't care about good/bad, they care about whatever strikes them as interesting or fun. Which is... a very mixed bag:)
The only danger in downloading something leaked on 4chan is that you run the risk of learning a little more about dolphins than you might have liked.