I read an interview recently where Oda said he didn't create Vivi with the intention of her joining the crew, but after seeing her popularity, he considered it, and in the end decided against it. I'll try and find it and post it here.
Which was he made Ms Wednesday to be a random villain, drew her with her hair down on a whim and said “hey, she looks like a princess” so she became princess of Alabasta
As far as the nakama thing, Vivi being a nakama implies that she's one of the strawhats, which she is. She's an honorary member, as established in the panel you posted. The translation of shipmate implies that the next time they meet she will literally sail away with them, which is not what nakama means. She can't actually fully join them since sge must rule Alabasta, but she will always be considered one of them.
The point is when Vivi reencounters the Strawhats she’ll be one of them again. They’ll meet up, they’ll sail together and she’ll be a Strawhat.
Yeah, after all the chaos of the final war is over Vivi will go back to Alabasta, but for the grand finale she’ll be a Strawhat.
This panel has existed for 20 years, and only in the past two have people tried to retcon it into a setup for Vivi to join the crew.
It’s not a “retcon”, is people reacting to new information and seeing what it means. Before we didn’t know how or when Vivi would reenter the plot, or how big a deal Oda would make it.
Now we know Vivi’s plotline a pretty massive deal, her dad’s been assassinated, she’s on the run from the World Gov and Zoro had to talk the crew out of invading Marijois simply on the off chance Vivi was there.