@ Kma:
You forgot that Thriller Bark is a WANDERING ISLAND, which came straight from West Blue (#443, told by Brooke ), so it's not possible to be a stationary guardian for the entrance to NW or such things.
guardian could refer to guardian of the secret that lets all of these undead wander around. perhaps its some sort of undead army plotted by Moria.
anyways, i was just offering some postulation. it is possible that Moria is some big dude with a bunch of subordinates just like croc and that this "guardian" is just one of those subordinates. at the same time, i dont think it is outlandish that multiple shichibukai would brandish a sword or specialize, at least in part, in sword combat.