Kill cam shows player not being affected by concussion grenade; is this a perk or a hack?

While playing Call of Duty Black Ops (original PC version) there was a very high scoring player that appeared to be unaffected by my concussion grenades. I would throw a concussion grenade right in front of him but then he'd kill me instantly after. The playback on the kill cam shows he was completely unaffected the grenade. No slowing down, no disorientation, no ringing on the ears; it was like there was nothing done to him at all.

I know the Flack Jacket perk will make you stronger against claymores and other explosives, but I don't think it blocks the effects of concussion grenades.

I do not believe lag was the issue as the playback showed the grenade exploding right in-front of the player, which is alway what I saw when I threw it. My ping on that server is always in the low 20s - 30s and the other player never appeared to lag when I saw him or her.

Is there a perk that blocks concussion grenades or was this player likely invoking some kind of cheat or hack?

To address n_palum's comment, I'm not complaining about "losing" as he/she suggested. I run my own CoD server and would like to know how to identify hackers and cheaters. "Is there a perk that blocks concussion grenades or was this player likely invoking some kind of cheat or hack?" is a perfectly valid question and I believe it adds value to this site.