Yeah achievements are messed up for alot of people. Its not uncommon. Anyway the guy provided proof of his kill. Its up to you to disprove it...which you haven't so far. I just hear alot of assuming.
You are a funny man. I was watching him exploit gear drops lastnight. What a cool guy Yep its an unintended mechanic that will be fixed but hes doing as much as he can before it is...