The statement of once means that only one other.
And no point it rules out Oden. Which was most likely created FAR before this point. Because something you absolutely do not take into consideration is that in the next 10 chapters Punk Hazard begins. And with Punk Hazard it's also the entire YONKO SAga that begins and with that it's the introduction of MOMO and with that it's the setup for the reveal of Oden in Zou and Momo's path.
In other words, you are completely ignoring the fact that when Oda drew this :

He would have had the MAJOR plot points of Wano and therefore the flashback of Oden already planned out for a LONG TIME before chapter 648.
Looking at the structure of the Saga, it can't be done otherwise.
So Oda knew that Oden was present on the boat already when drawing this chapter. (and he surely also knew that someone like Oden was present on the boat already back while writing the end Skypiea.)
Dude stop trying to defend a clear retcon. Oda doesn’t write a perfect story
2. There is no retcon here. Again, you are missremembering and missunderstanding CLEAR statements in the story, and riling up a criticizing for absolutely no reason.
And the problem is that it's the same thing for 99 % of the critic against One Piece on this forum.