either registered as a throat shot or shit happens its arma 3 its buggy. ive had several people eat mk1 headshots before. lag has nothing to do with it
funny thing about arma and lag and damage registration. if you hit on your screen you hit end of story lag may cause them to die later than expected but if you hit you hit. even if on their screen they were behind cover for 10 seconds if you saw you hit it registers. so its not lag that made him survive he just tanked