Ok, let's all break this whole argument up.
Very clearly we see new armor, there are four different explanations going around:
1. It's disc locked content. One of the biggest criticisms is that hackers haven't found it. However, from how it sounds they haven't found everything yet. Perhaps it is only available after Bungie hits a master switch, in which case hackers still wouldn't have access to it. When talking about the Multithreat chest piece, Bungie did say it's only one of many pieces set aside for this sort of thing, and Bungie has hinted at other milestones like the one needed to lift the rank cap.
Plus, according to some questionable intel, there was no armor with the map pack. If I had to guess that supports disc locked content as Bungie can hit the switch the day the map pack is released. I don't think Bungie is one of those companies to sell content on the disc as DLC and am pretty sure they would just do an autoupdate instead. In fact, I'm pretty sure they need to do an autoupdate to update the achievements.
2. It's DLC. One of the biggest criticisms is that the way the engine is set up makes this very prohibitive, if not functionally impossible. Either Certain Affinity found a way to make it possible or Bungie lied to us, I have a hard time believing they wouldn't implement a system to allow DLC armor when there was demand in Halo 3, however, they have said it can't be done practically.
3. It's a dev build of the game. I highly doubt this. Of all the players, only one is seen wearing armor other than default. This seems typical of the Halo hype wagon, a breadcrumb trail of clues. However, the biggest flaw to this theory is they aren't going to introduce new code that would not be shipped during the map testing. If you remember to the dev process of Reach, the last month of development nothing could be added, it was all bug fixes. Anything that would've been added could have potentially destabilized the code more.
I am near 100% certain that Certain Affinity used the latest build of the game instead of an earlier dev version. Obviously they would work on the same version of the game we have or a newer version we'll be patched to so that testing can replicate our conditions as well. Using an older version, digging up cut assets, or adding new assets just wouldn't be done as it could be unstable and throw off testing.
4. It's fake, people are mistaking EOD or Gungnir armor for the helmet and the Assault/Breacher for the chest. First of all, the two visible pieces (helmet and chest) have minimal resemblance to the aforementioned pieces. Indeed, they have characteristics that distinguish them quite a bit. And of the rumors that Major Nelson decided to spend a few hours on Photoshop before uploading them to Waypoint. Again I say we're talking about Major Nelson, we're talking about the official Halo site. Major Nelson would have to be the biggest troll in Halo history to do that, not to mention there'd probably job repercussions were he to do that. It's not shopped.
Conclusion (TL;DR):
Through invocation of Occam's Razor, I have to side with the disc locked content people.