Is the flashback about haki? Nope, it is about fishman karate and Sabo fight skills with she use in combo with HAKI.
In PH Robin clearly not have Haki, same for DR.
Bro it is shonen though, Naruto with Kishi, Kubo with Bleach, sadly their are many example of character getting far stronger, the usually logic in manga overall.
The point is, since DR 2months pass and we as reader didn´t see how Robin improve like the other strawhats.
Oda focus to much to Luffy training, he should at least show one chapter where all strawhats training together.
But he never really did it, even in pass he didn´t did it outside of Luffy.
So im not mad at all, 2months are enough to gain haki for Robin.