If i recall he was ex JDF where he did some kinda special forces thing and then was a drill instructor and then an instructor for the equivalent of SWAT teams. My source is what I remember from a magazine interview I read in the mid 2000s.

I think theres an old video out there of him demo-ing the snake eater CQC style that he made up and while it was disguised in 'you would need to be super extremely skilled to do this stuff', the subtext seemed to still be 'holding a knife and a gun at the same time is stupid as hell but looks cool and I had to make it seem practical'.

I also love im MGS2 how as soon as an alert or evasion is triggered the guards go from video game obstacles who walk back and forth in set patterns to a force to be reckoned with that acts pretty smart and even sometimes surprises you.