My late brother was ex USMC sniper followed by career SF Weapons specialist including "target interdiction" specialization (mostly "middle east"). The figures are extreme but possible. The prior CDN record was achieved with "borrowed" USA ammo pre-heated in the sun. Also don't forget these ranges are a team effort with the spotter who is actually directing fire. They're rarely one shot one kill and rely on inept target response. 3 rounds is not unusual for a kill at those distances.
Jeremy