Considering the first kill spawns the ???, i'm only guessing that only the first killshot gives the NM title. You can see it on the wiki under "NM Title".
Dynamis - Tavnazia - FFXIclopedia, the Final Fantasy XI wiki - Characters, items, jobs, and more
I got killshots on two of the Diabolos, one on the second and one on the final aspect, and my title is "Dynamis-Tavnazia" Interloper.
My guess is the title went to whoever got the final hit on the first Diabolos kill. I'll have to find out who got it last night.
XI Wiki

