The link had a really interesting but unsatisfactory explanation, IMO. The way I am understanding it is as follows: Man A and Man B are traveling (with combined speed of 0.866c) away from each other. Because of time dilation, to A it appears that B's clock is twice slower (1/sqrt(1 - 0.866^2) == 2). Then A shoots from some sort of weird space gun whose bullet is instantaneous. The bullet passes B at 4 seconds (relative to B). B notices that and shoots back, and B's bullet, due to equivalent effect hits A at 6 seconds causing a dead grandfather paradox.
However what I don't understand is this -- to say that the bullet shot by A passes B at 4 seconds presumes that we are talking about A's frame of reference (because it is from A's point of view that B is 4 seconds "behind" -- and likewise A is 4 seconds behind from B's perspective). However, what if, from B's frame of reference, A's bullet actually passes B at 0 seconds?
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.
However what I don't understand is this -- to say that the bullet shot by A passes B at 4 seconds presumes that we are talking about A's frame of reference (because it is from A's point of view that B is 4 seconds "behind" -- and likewise A is 4 seconds behind from B's perspective). However, what if, from B's frame of reference, A's bullet actually passes B at 0 seconds?