I was thinking something similar recently....I think it was more like, after his experience following the Young fight, he was so happy that he wanted to concentrate on preaching. He was 26 then, Not fighting for the next 4 years or so saved him from punishing, brutal fights against a peak Gerry Cooney, Mike Weaver, Michael Dokes, Greg Page, Tim Witherspoon , Earnie Shavers, Gerrie Coetzee, Trevor Berbick and, quite possibly, Larry Holmes. Dont get me wrong, I think he wins most of those fights if not all, with the exception of the Holmes bout, but they would all be tremendous bouts and, at least Shavers would not go down in the first three rounds. And so by not fighting all those years, when he returned, his body was really that of a 26 year old, punch-wise talking. That might be why he boxed so well for ten more years.
If it's the Spinks that' beat Holmes in the first fight, then he's got a good chance of getting a decision. Tommy Morrison beat old G , so Spinks definitely in with a chance.