It's the author's excuse to make characters magically stronger throughout their adventure because the plot demands them to fight character x or y, you can basically call it whatever you want - it is basic battle shounen stuff, and OP ain't an exception.
We had this discussion in the last thread before, but you forgot to answer i think and then each of our participation stopped, and i think you have not understood my point.
Zenkai boost is an increase in base lvl power by means of fighting and getting injured and then recovering from it, that´s it. Basically your base power level increases whenever you fight seriously and recover.
Means the author using this device does not have to underline characters getting stronger, it is implied since they regularly get this boost, fight seriously, get stronger. Meaning not new techniques and so forth, just assume a basic punch is stronger than before.
The OP equivalent would be your Douriki getting a significant increase from each fight, and that´s it.
Robin getting Haki, with whatever means she got it, is the opposite of a Zenkai boost. We don´t have to assume anything, it´s the author telling us, Robin is stronger than before, she has received a power-up. Whether that´s through training in the 2 months, or awakening throughout the fight, is secondary. We have clear indication of it being so.
Just to give an example, Zenkai Boost would be to assume that Sanji´s Anti Manner Kick Course is stronger from Alabasta to Skypeia, and again from Skypeia to W7, and again from W7 to Enies Lobby and so forth, without any indication of it being so.