Difference was that he was not on drugs. No doubt he drank some but I think that overall, he kept his wits about him during those years “off grid”.
Certainly not on drugs or alcohol but much of the "danger in the forest" are from those suffering from mental illness... and the duration of his survival was a heaping load of "serendipity", too. Kudos to human physiology and adaptivity.
Specifically, the bodies innate ability to adapt and compensate during those long winters and his serendipitous ability to essentially "build up" his "reserves" in the fall, and "hibernate" in the winter. Plus his ingenuity constructing shelters, use of camoflauge, observation, and escape routes. I'm sure this is already influencing @Gallowbraid to develop plans for his "disaster-dependent forest chalet".
Definitely a "whack-job", or unusual manifestation of Asperger's Syndrome, but certainly inspiration for those students of "post-apocalyptic survival". Now I'm left wondering which Hollywood producer has negotiated options on this story for future production...
Here's a link to the story, reprinted by GQ... https://www.gq.com/story/the-last-true-hermit