For anyone reading this, the referenced explanation finally did make sense after I drew a Minkowski diagram of the two duelists. I guess this means that I finally understood special relativity... Brief summary: FTL would indeed violate causality, unless a special frame of reference is proposed (this violates special relativity) or a specific new property of Universe is established such that it would prevent certain events from occurring (aka Novikov conjecture) -- and we don't have evidence for any of the two being true.