It's not about whether they are lying or not. It's about the reliability of their statements. For example, one of my friends was involved in an incident on a race track. The driver in front of him decided to take a cut through on the racetrack for some unknown reason. Anyway my friend smashed right into him causing the guy, who took the cut through, to barrel roll out of his vehicle. Now there was unquestionably contact between the two of them. Not my drivers fault at all.
So anyway, this happened on the other side of the circuit where there happened to be a group of people on some Segway experience thing. Why they were there I don't know, some team building bollocks. Anyway, I asked them if anyone saw the incident. A couple did and do you know what they said ? "the guy just flipped on his own there was no other kart anywhere near him".
They have NO reason to lie... however their witness statement was TOTALLY incorrect. Witness statements are not statements of fac, no matter how good intentioned they may be. I learnt a valuable lesson that day.
This is why the Police want to take a look at the video (if there actually is any video which I doubt.)