This isn't the first rodeo for Anthony and Joe Russo so they wouldn't post a spoiler to their instagram. They like their jobs and they probably don't want to get fired. There is no faster way to lose your job than to break an NDA which we all know Marvel probably has everyone sign one that is no less than 500 pages long. That didn't stop every other outlet in the world from reporting that they had just spoiled something huge from Thor: Ragnarok or Avengers: Infinity War.
The picture shows Mjolnir despite the fact that we saw the legendary hammer destroyed by Hela in the first Thor: Ragnarok trailer. The actual hammer didn't get destroyed though, the prop still exists, and it wouldn't be the first time someone takes a picture with the hammer. It's the Marvel version of holding up the Leaning Tower of Pisa.
Despite the fact that we saw the hammer destroyed in the trailer there wasn't anyone who really thought it was gone for good, right? The hammer has been destroyed many times within comics continuity and it always comes back. It's part of Thor's schitck just like Iron Man's suit or Captain America's shield.
Thor: Ragnarok will be directed by Taika Watiti and will be released on November 3, 2017. Avengers: Infinity War will be directed by Anthony and Joe Russo and will be released on May 4, 2018.