Cool. No link backing that up. Apparently the french version of SBS 73 says Sanji is the left hand on the crew, but I couldn't find enough evidence to bring it up. So you need good evidence to back up that statement not being mistranslated. Looking it up, I just found kazuki.i on OPforums mentioning it.
And as I said before, right and left hand is determined by strength. That's why in every walk scene, Zoro is closest to his right, and Sanji is closest to his left. In every full crew vs full crew fight they've had, Zoro always takes on the right hand and Sanji takes on the left hand. I put Robin above Sanji for significance to the crew, but that didn't give her the left hand status. Zoro sleeps and is the right hand because he is the 2nd strongest, but Sanji cooks and fights the 3rd hardest and isn't left hand? You know how much sense that would make?
Sanji becoming the de facto captain of the crew with Luffy and Zoro gone settles the entire debate, since Nami was there and was the one who named the crew name after Sanji. That doesn't even need a translation check or more than 1 person talking about it like the internal captain you mentioned from the yellow databook, and the french sbs 73 volume I mentioned. It is just plain obvious facts that happened.
You can say Nami is more important than everyone but Luffy. You can say Nami directs them on the sea, and give her as much significance as you want. I disagree, but can find reasoning behind it. But right and left hand doesn't belong to anyone but the strongest. That's how every crew works. In every fight, Nami goes to the back. In most cases, even Usopp fights stronger people than her (which needs to be fixed imo, because its time for her to shine in combat).
As I said earlier, nobody in the OP fandom calls Nami the left hand, because it has always been measured by strength alone. A quick google search of "Nami left hand" will show nothing. That's why I'm so fascinated by this debate. It's the first time for me. I'm a Nami left hand debate virgin. We are the first 2 to ever talk about Nami being the left hand (probably not really).
He didn't "mention it", he highlighted important facts from each of the 5 databooks. There were multiple things listed across multiple different books, regarding multiple different topics. Meanwhile, if you actually looked at the content of SBS 73, you would see that nowhere is it listed that Sanji is the left hand. It doesn't even mention anything near that. So what we have again is are two instances of Oda specifically saying that Nami is more important than most everybody on the crew, disregarding Luffy and Zoro.
He has said that Luffy, Zoro, and Nami are the triggers of One Piece. He has said that Nami is the internal captain of the crew, in the same databook that he acknowledges Zoro as the VC. And then on multiple occasions, based on the way the story is written, he has expressed Nami's importance being paramount to most of the crew, besides Luffy and Zoro. Like we even got a moment where someone from the PK's crew complimented her and told her how important she was. The PK's crew.
The idea that you have where strength determines position is blatantly false. Not only is Nami more important than Sanji, but she was there before Sanji, is acknowledged as one of the three triggers of the story, and when she speaks, everybody including the Captain and the Vice Captain drop what they're doing and follow her orders immediately.
Sanji didn't become the "de facto captain" of the crew at any point, you're only exaggerating a moment that on Nami's part was comedic. If Sanji was the "de facto captain", he wouldn't have asked Luffy for permission, he would have done it regardless. After that moment, there was nothing that indicated that Sanji was above any other person in that crew. It isn't fact's that you mentioned, it's twisted interpretations because you're a Sanji fan and try to make him more than what he is.
Luffy's crew has never, ever been based on strength. Luffy is the most important because he's the Captain. Zoro is the right hand because he's the next most important as the Vice Captain, and the next in line as the 2nd member of the crew. Nami follows Zoro as the left hand, because she is the Navigator, as such the next most important, and was the 3rd member of the crew.
By the way, if you want to mention facts, Sanji himself disagrees with you. Consider for a second why Sanji would call Nami "san" and Robin "chan". It's seniority in his eyes.
It has nothing to do with strength. Oda himself on two occasions, the story itself, the characters themselves entirely disagree with you. Be fascinated all you want, it doesn't make you any less wrong.