After reading the article I fully believe that this is the ultimate end of Jimmy Hoffa and how it came about. I think that finally the man that pulled the trigger is known and given the honor code within the mafia, if such exist, the real source of the contract won't be known but could be figured out easily. Either way all the players are dead now and there will never be justice for the actual Murder.
Hoffa lived by crime and died by crime which happens to most mafia people. I think most likely Hoffa never know what happened but most likely was like the ending of the Soprano's TV Series where everything just faded to black. Given that Hoffa was ended by two head shots he most likely never felt a thing or knew anything of what happened.
The most prevailing evidence that convinced me that the guy was telling the truth was when they determined that there was blood stains into the wooden floor and even though a positive DNA match couldn't be obtained given the time that had passed I'm confident in my mind that this was the actual scenario that played out.
I did though find it interesting that the killer tried to send a subtle message to Hoffa that something was amiss in the way they set in the car on the way to the house but as far as he knew Hoffa may have had his own suspicions and being nervous, if he was, may not have picked up on the clues that the killer was sending him. That's the way it works in the Mafia though your killer may well be your best and closest friend up and until the friend is made an offer he/she couldn't refuse.