Either it is completely fake, made up by the navy to hide something else that they wanted to cover up, and could also serve as a confusion tactic for other countries aerospace programs, some sort of fake internal thing to tease out a mole or another situation like that.
The other possibility is that it really is some sort of new propulsion device being tested by a top-secret government agency or some other private party, and somehow they made the fatal mistake of being sighted by the jet. I simply dont believe the us military would release this video if it was actually a new secret technology they were trying to hide.