BStrong
Penultimate Amazing
It has nothing to do with the assassination.
The term "Exceptional Clearance" describes an established process used by law enforcement agencies and prosecutor's offices to come to a finding of fact where the accused is not available (death of the suspect usually, but it's also used in cases where the accused has escaped or absconded) and it's that process that was used in LHO's case.
If you'd like to dispute any part of the four conditions specified as criteria by the FBI at the link I posted as relating to the JFK assassination I'd entertain that discussion, but I don't believe that you've ever encountered the concept before and may need time to investigate it for yourself.
Might want to look into the concept of trial and conviction and sentencing in absentia too, as it's a judicial cousin of EC.
In the meantime, "EC" explains exactly how LHO is named as JFK's assassin w/o benefit of a trial and conviction.