HELL YEAH I GOT it, my god it was painful, the last kill was obtained in afterlife too!
Apparently yeah, Dumien was also surprised it counted. Unfortunately, theater doesn't do a good job showing what happened, but the camo challenge award thingy that appears on the screen upon completion was rolling when I respawned. I would've recorded it, but since I was getting so frustrated and didn't expect to get the achievement done in any special manner, I wasn't recording. What a missed opportunity that was.