Which only means one thing: Half-Life 3 confirmed.
Is this enough proof that a Majora's Mask remake is in the making? The answer is both a yes, and a no. Remember, this is Nintendo we're talking about: a company known to always port/remake their N64 games eventually years down the road (namely: Super Mario 64 DS, Ocarina of Time 3D, Star Fox 64 3D to name a few). I would be daft to say that they'll never remake Majora's Mask, because Eiji Aonuma has been hinting at something, but to say it's in development now? No. Look, we've already got TWO big Zelda titles this year (not counting Oracles) and a big TBA Zelda Wii U title is currently in development for next year- that's a lot of Zelda games to make in a short amount of time. Granted, Wind Waker HD was made in only 6 months, but mind you that Majora's Mask is a completely different game made 13 years ago when 64-bit was the norm, so they would have to build the game from the ground up rather than just slap HD paint on the characters and call it good. If we want the game to look as good as this, it would take a lot more time than just 6 months to develop. Furthermore, the game was objectively one of the hardest Zelda games, and knowing Nintendo, the game would have to be fine-tuned into being more user friendly by having more save points for one thing, and I'm sure they have to have Tatl repeat "Please take a break" ten-thousand times at every opportunity.
On the topic of the Smash Bros. screenshot it's ridiculous people even translate it as a MM remake teaser, I mean it's a game that includes the entire Nintendo universe, of course it would reference Majora's Mask. It doesn't mean it's teasing a new game. It was the same ordeal with the mundane rumors of the Pokemon Ruby/Sapphire remakes being made, because Pokemon X/Y would mention the "Hoenn Region" as if the series hadn't already referenced past regions many times before. With references, the game remains canon with its series; it's not put there just to tease everyone to believe that there's definitely a new game or remake coming out RIGHT NOW. The Zelda games have been referencing each other for years I doubt it will end any time soon. I'm not implying we'll never see Majora's Mask in fruition, but now seems too soon.
I could be wrong, so don't mind me if you don't agree with me, but just don't be disappointed if your crystal ball is malfunctioning.