Having watched a ton of aircraft test footage from that era, I disagree. And given what I know of govt management systems, I firmly believe someone wrote over the wrong tapes.
To clarify, you have seen aircraft test footage that shows strings jerking the aircraft up and back at odd angles and have seen people write over the only existing copies of mankind first achievements in aviation (i.e. first hypersonic flight)?
Because I would say 1960s test aircraft footage being grainy/jumpy and overwriting old footage the 6th test flight of the umpteenth hundred aircraft ever developed aren't exactly the same.