The fact that the series was revealed to be a documentary of sorts neither added or subtracted to the whole for me. The revelation just felt gratuitous, almost as if it was uttered only in order to have a diagetic reason for the crew photographs that flash in the final seconds.
I don't know why you're emphasizing the fact that the series is often narrated in the past-tense, Omaha
. This fact informs us that the characters are recounting their story after the fact, but it doesn't predicate that the entire series had to have been a fictional recreation. It strikes me as an unneeded layer of fiction that serves no real purpose except to provide one last "gee-whiz" to the audience.