She wouldn't have been able to do this if Hakuba was too fast for her to react to as Hakuba's position is currently changing.
Man, that's why "running straight at her" is the key aspect. He ran straight at her, didn't try changing direction or anything.
Barto went from unable to perceive Hakuba when he was attacking him at different angles in close quarters, to clearly seeing him running at her when he ran straight. The difference is quite apparent.
The panel please. The scene I have in mind didn't parse as "several kilometres away" and I want to crosscheck against the panel to be sure I'm not misremembering.
I'm not at home, can't post
It's at least a couple of kms. Hakuba was some distance away from the base of mountain. Robin was more than mid way up that mountain.
Dude covered that distance within seconds, his speed is top tier. There are only a handful of characters in the series so far who are as fast or faster in pure speed/mobility (just Kizaru, Enel, speed mode King, Sanji and G4 Luffy off the top of my head)