I mean, why wouldn't you be able to form a contract through tweets? You can form a contract through any medium you want, especially just to give permission to Disney to use your tweet.
I don't think Disney is setting any precedent or anything else weird by saying "send us messages you want us to publish, by the way here are the specific terms we want to use" on Twitter, they've been doing that for a long time before twitter even existed. Think about America's Funniest Home Videos, they did exactly this for decades but over TV.
Your tweet fails to give Disney any "consideration" so it's not going to be a valid contract and not because it's in a tweet. Offer them clearer terms and a dollar and then maybe you'd have a point. Right now they offer you a chance to see your tweet in some marketing materials, better than nothing right?
Lastly, this is incredibly specifically scoped. Does anyone tweeting "Love the new movie #MayTheFourth @Disney" really expect Disney not to want to use it? They are going to have someone review the tweets before they use them. How could this possibly come back to bite Disney?