As absurd as the single bullet theory is, for this guy to not come forward for 60 years strains credulity about his story.
It's perfectly plausible that he didn't know the significance of his recollection of events that day until later on. He says he suffered PTSD and didn't pay much attention to the investigations or alternate theories until the 2010s because the memories were too painful. Still, the fact that he waited so long to come forward with this information definitely makes his story worthy of skepticism.
More troubling though is the fact that neither the FBI nor the Warren Commission interviewed Landis. The trend throughout these investigations is that witnesses who had information that conflicted with the Lone-nut theory, were either ignored or were pressured to alter their testimonies.
Vanity Fair's article is much better written than the NY Times article on Landis FWIW.
I personally think he's telling the truth. While it's possible that he mistakenly put the bullet on Governor Connolly's stretcher thinking it was Kennedy's stretcher, his claim that the bullet was found in the rear seat where Kennedy was sitting, not the front seat where the governor was sitting, is not something that is easily forgotten or misremembered.