As a psychology major, I learned that false memories are incredibly widespread. The duration of time between the incident in addition to specific communication cues can lead to the creation of false memories. This is especially seen during cases of childhood sexual abuse (does the term 'repressed memories' sound familiar?).
In Williams's case, his experience in being the helicopter an hour after the crashed predecessor could have been influenced by the his experienced emotions and those around him who may have posed questions such as 'Are you scratched from the accident?'. (Note the language which implies that there was an accident and is thus asking for a pseudo-confirmation under the assumption that he was present.)
He has also been a pseudo-celebrity. I caught a taping of Williams at David Letterman in October 2014, and was struck by how he was sharp enough to maintain an interest and knowledge in a wide expanse of topics. Such intellectual rigor is always demanding on the mind.
So we'll see if anything changes if or when he comes back. Until then, it's Lester Holt in the spotlight.