That's just Nintendo's opinion on the matter. This letter is just them asking Valve politely to please take down the emulator.

Until this stuff actually goes to court and an actual judge decides on it, nobody knows what the truth is.

Who am I kidding? Even when the truth is known, they'll still abuse the expense associated with the legal system to bully people into submission. Sony sued a commercial emulator developer decades ago. They made the asinine argument that the screenshots they used was copyright infringement. The judge said it was just comparative advertising instead, and that it was actually good for consumers. Nevertheless you still see these monopolists take down emulator screenshots of their games as if they had the right to do it. They know they won't fight back.