Me and some friends spent over an hour killing most likely over a thousand of them, contacted MA support and he said it's time based like the other ones and we should see notification in chat when it's happening. I have yet to see that notification, and your comment seems to point towards this being false.
Could we get a progress bar or something? Maybe the amount needed to be killed lowered? Or are you stating how it's intended to work, when in fact, it is bugged?
I would guess the support meant that the event has a certain cooling-off period and maybe you tried triggering it during the cooling off. As far as I can remember, these were never time-triggered, maybe I am wrong.