From what I've heard the anime was really starting to ride the manga's ass raw at that point. They were airing Robin's flashback at around the same time Zoro defeated Kaku in the manga, I believe.
Actually, if I recall correctly, Robin's began in the animation right around the time it was revealed Cocoro was a mermaid.
Your point absolutely stands though.
It was incredibly close….
BUT
It was still fucking awful.
And there's simply no excuse when they have PLENTY of material to work with in side-stories that are 100% canon and relevant to the actual plot.
Remember the episodes where they jumped off the roof and one episode was a FB for each character about their life?
Yes, yes, it was to catch people up in the new time-slot. That doesn't mean it wasn't terrible.
EL and TB sucked.
Compared to the WORST episode of those days, this episode was like some Ghibli shit right there.