Not having checked his data on the plants it does sound plausible. McCandless *was* an incompetent dreamer (no matter how noble the dream), and anyone who has looked at the caloric contents of wild plants know that you have to gather a metric buttload (the imperial buttload is slightly smaller, and insufficient in this case) in order not to starve. If he had killed one or two moose and done some fishing he could have lived well for a year, assuming competence in taking care of this harvest.
I suspect that McCandless suffered from White Guy Advantage Syndrome ("I'm an educated white guy, so I can quickly and effortlessly master skills that it takes years for the natives to learn", very common in movies and certain types of stories). Hunting at the level he needed is not easy (compare the Norwegian Kristoffer Clausen, who brings years of hunting experience to his experiment, a hunting dog, both a shotgun and a rifle, and *still* has lost quite a bit of weight).