I think 50 years is an exaggeration. Maybe 20 years, but not 50
We know a little about the Aurora project and it's advanced pulse detonation engine, and these things may have came out of that same skunworks area.
I wouldn't be surprised to learn that there is technology developed by the military in the '60s that we still don't know about.
20 years is your figure, but the common one is 50 years - also quoted by notable people in the secretive aviation community like Kelly Johnson and others who worked at either the Phantom or Skunkworks
Actually, we know absolutely nothing about the Aurora project beyond its name and even that is suspect. All else is internet speculation.