It's possible to survive the jump from an aft door like he did....the problems are that it was night, roughly -50 degrees at the altitude at which he jumped, had inferior skydiving gear, etc.
The flight crew said he was wearing a trench coat and loafers, which led investigators to believe he couldnt have survived the jump, much less the terrain in which he landed in. However, before he jumped, the entire flight crew was locked into the cockpit...so who knows what kind of equipment he had on the plane....the inferior skydiving equipment that the police provided him in his Seattle landing may very well have been a diversion to lead FBI officials to believe he never could have made it.
I think he made it, and got away scot-free.....the only trace of him was a few thousand bucks washed up in a river (again, another diversion possibly). In the 40 some years since the jump, SOMETHING would have turned up...skeletal remains, parachute, money, etc. Nothing.