No they explain this quite early, what they have to go on is mostly available video for major events but for individuals, they rely mostly on social media. We all know how honestly people portray themselves on social media. Which leads us to this:
The director is not totally omniscient, which become quite clear after the first episode. The director works with the information he has available, which is flawed to begin with. They are basically doing the best they can because they have no other choice. You work with what you have, then you roll the dice and take your chances.
This can be explained through to some degree quantum theory. We do not know enough to know if they are sending consciousness back in the same time line or if when they do send consciousness back, it goes back to an entirely different quantum timeline, one that is perhaps created by their very actions. If this is true a new time line is created with an entirely new future but the old time line still exists. If you believe that timelines are linear and set, then when they send people back it will not affect their timeline because it never did. Great now my head hurts.
The end makes sense if you accept quantum timelines. The timeline their consciousness is in now is a completely different timeline with a future they have no real idea about because it is completely different from the timeline from which they were sent.
If you are stuck with thinking of time as being linear, with one timeline that they are changing, then it definitely does not make sense because of paradox. Quantum theory solves that paradox.