My evaluation of the rumour:
+ It's totally plausible that something like this could leak from the E3 show floor
+ The would-be source of the photo isn't someone claiming to have secret insider info, they're someone claiming to have seen something they weren't supposed to have yet.
+ The banner, name, and concept do not seem like the typical kind of invented fan nonsense; it's not Zelda: Curse of the Crushed Bone Skull Warrior or whatever.
+ The idea easily interacts with NFC, and fits well with the idea that Nintendo is having a roundtable about a game they're announcing
+ It sort of follows up the NES Remix and I guess reflects a kind of direction Nintendo's been poking at recently
+ McWhertor is presumably saying he has confidence because he knows this is in fact real, but it's not clear how he knows, because if it was from some sort of long-lead pre-E3 event, he'd presumably be NDA'd and commenting on this would probably not be ideal
- It is clear to me that the original site just ran whatever got sent to them, and did absolutely no due diligence in checking the source or asking for followup
- The photo is low quality as hell, which is a classic hallmark of fake stuff
- The other banner images, eg Pikachu, look a bit like stock art and not as much like banner art.
Out of curiosity, has anything from Nintendo been leaked from a banner a few days before an E3?
Yes, New Super Mario Bros Wii. Someone posted photos of Nintendo's section of E3, all under thick drapes, but the shape of the shadows visible under the drapes was unmistakable and people (I'm pretty sure our very own Crushed/EmCeeGramr) identified it.