I just played a team snipers game where a guy on the other team was pulling off instant headshots the entire game. During all the killcams he was scoped in and as soon as you would come out from behind cover his reticle would instantly move to your head and kill you. He finished with like 38 kills or something insane like that. I mean I've played against good players, but I've never seen anything like that.
It's entirely possible. If you legitimately think someone is cheating just report them.
I ran into a guy who would shoot the AR faster than the AR shoots if you hold the trigger. Get the jump on him, he started firing, won the shootout EVERY TIME. It happens in other games too but yeah. Cheating happens.