Robin didn't have haki before this arc. This is just a fact, with many examples. Could she have awakened Haki(like Usopp in DR) in Wano? Why not? This is a natural level-up. Another thing is that it's all poorly written. Oda did not give hints that she would have a CoA in this arc. She just showed up at the right moment.
We have examples of awakening Haki, with CoO(Koby, Usopp). It's a pity, I wanted to see how it looks with CoA.
I still hope, Oda show it with Usopp\Nami or Chopper\Brook.