I have a hard time with that explanation, though, because it doesn't make sense to me that they could be so far out ahead of non-governmental aeronautics.
Well, you'd have to believe that exotic crafts are flying around our planet, and that they were downed and/or have crashed at points in time and been collected by the USG.
Then, you'd maybe believe that during Eisenhower's administration, these were turned over to private companies, who made breakthroughs of their own.
Whether they fully informed the USG of these breakthroughs and the extent thereof, who knows.
One of the explanations I've heard for why the technology hasn't leaked is that the people working on it don't understand it well enough to replicate it, but they're working out how to control the ones they have access to.