Well, apparently this challenge (kill all survivors by any means in a single round) triggers as being completed if some survivor disconnects mid loading into the game; when game backs you off to the lobby.
So that probably means that if someone disconnected but you killed everyone else it would stil count.
You don't need mori for this. Just 4k.
Where did anyone mention a mori?
The point of this topic was to inform that if someone disconnects before the game even loads in, the challenge will be completed, as if killer just 4k'ed, even though killer didn't even move their character at all during that match.