From Wikipedia: "The radio program This American Life reported in 2009, that, contrary to claims made in the case, the accident report contained no information on the secret equipment on the plane except to note that secret equipment was present, a fact which had been reported in the press at the time. The program interviewed the daughter of one of the crash victims who described the government's claims in the case as fraudulent."
So, basically, you and a crash victim's daughter describe the government's claims as fraudulent. Great. Oh and probably a number of plaintiff lawyers on the victim's side when the whole mess was retried too. But they're always claiming stuff is true right up until it's proven false in court and then they're "terribly disappointed" by the verdict.
Bottom line here is that the case was retried because a bunch of people were convinced the original claim was fraudulent and STILL some FIFTY years later, the courts found the claim to be valid. It was appealed and the District Court found they were valid too. Nobody had any dog in the fight any more at that point. So if the claims still seemed valid to those guys that specialize in such matters, I'm inclined to believe them. There was no fraudulent claim to privilege at the time and we need to stop yelling there was. Even if you disagree with the decision, you have to accept that there's enough gray area in this case to warranty a little restraint in the usage of it as an example everywhere.
However, the cause of the plane crash was determined to be a fire in the engine. What does a fire in the engine have to do with secret surveillance equipment on the plane? Why would an engine fire be privileged? How would its disclosure impact national security?
No...not the cause, the report itself. They didn't want to release the report. It contained broader information that they didn't want other governments to know at the time. It had no specific secret equipment details, but it did have mission details that (when considered 50 years ago) were secret. Read the 2005 appeal section of that wiki pedia article you quoted. And keep in mind that the original plaintiffs in the case were offered unrestricted access to the remaining survivor and turned it down. They could have surely gotten the information they needed about the fire had they just called that survivor to the stand. But, no, they wanted the full report released and then everything got all hung up at that point.