Well, you're playing on PC, where it's easier to land headshots.
The player is unknown because you left the group and their names aren't being tracked anymore.
Depending on the mission, it would be possible to get that many headshot kills if there is a large amount of NPCs that spawn or you guys group wiped multiple times.
And with 65% accuracy, it looks like the player was running a Marksman rifle.
Really? Aimbot? That's a horrible aimbot, then. Headshots are easy as hell to achieve it's not rocket science to get one. Aim chest high and let the recoil go up until it hit's their head. This is common practice in shooters. Also, if they are at all running a Marksman Rifle, it'd be easier. (FYI, I play with a controller on a PS4.)