I think the reaction may be latent in all Elite combat Flood, but normally never triggered because aggression related behaviour dominates. If so, that could be a design oversight or maybe Bungie realized but just weren't fussed to address it.

: Interesting that it's cued specifically to the pistol though...
: wonder why?

The animation can be caused multiple ways, not just with a pistol shot, so I was curious about the term 'pistol' being there. But from looking at the file names for sounds relating to covies, I believe it's just a code word relating to the armed status of the subject. It indicates something like "armed with typical weapon or none at all". The point being, his armed status causes no additional sound in regard to movement (unlike the case of a Grunt with an FRG, for example).