She tried to kill Luffy with it (even though she had no intent), so I guess it's sharp or else she's stupid.
So wait…............. are you saying Rebecca can stab somebody if she has to...............b...b....bu.......but what about her fighting style?
--- Update From New Post Merge ---
Which is why Rebeca being the person that one hit KO'ed everybody sounds absurd, whoever did the damage can move so fast that the eyes can't track them unless you use Observation Haki also Sabo heard a rumor and Dressrosa has a rumor about things disappearing out of thin air, you put those two together and they make magic.
Sabo heard a rumor about speed. That's all that was said. It didn't say he used Observation haki to see it.
Does she knows Armament Haki and If she happens to know how to use it, I don't think it will strong enough to deal damage in gladiators such as Orlumbus, Cavendish or Suleiman and believe me If she were to start attacking people her attacks cannot be that powerful, we are talking about guys that have been in the New World for more than 3 years and have probably taken a lot of damage but somehow they're KO'ed by Rebeca who also happens to be KO'ed, be serious.
If she knows CoO, then her learning CoA isn't really that farfetched. The one you can never learn is CoC. Which is something you have to be born with. We don't know her actual attack power. She hasn't been hitting people. That's kinda the point of her current fighting style. It's to dodge, not to attack. If it was Rebecca, it would be the usual shonen trope of she's overused her ability and fell down after or she went Berserk and then stood after going "WTF, just happened?". Well, her statement will be more shonen friendly of course.