What the heck could Luffy have said that would cause that reaction? Especially since they already know what we already know, that he wants to be the King of Pirates. Which naturally Oda wouldn't have hidden anyway.
So Luffy said something else...
The boys are yelling out their ambitions in front of eachother. Ace goes first, then Luffy takes his turn and…
…Oda cuts away ...and cuts back to Ace and Sabo's reaction of confusion and surprise.
What the heck did that boy say.
First thing that we should be on the same page about is this: it wasn’t Ace that started shouting his ambition: it was Sabo. His ambition was to become a pirate, see the world in all its glory, and write a book about it.
Following Sabo’s declaration of his ambition, Ace then proceeds to state his own: to become a pirate, to beat every last person who stands in his way, and to earn himself the glory that dreams are made off.
In both instances, their desire was to become a pirate.
Following Ace’s declaration of ambition, Luffy then proceeds to shout his own. ‘Induction’ suggests that what Luffy shouted was his ambition to become a pirate as well, given the fact that such an ambition was exactly what the two before him did declare.
Also notice how he shouted ‘I’m gonna’. He did it with raised arms. That’s usually the stance that is drawn by the author when Luffy shouts his dream of becoming the Pirate King. And when he does so, he practically always precedes it with his famous opening line ‘I’m gonna’. So these are good reasons to think that what he shouted was his dream of becoming the Pirate King.
Luffy’s brothers then react in great stupefaction at what was said. One of the things that would surely cause such a reaction is his declaration of his dream to become the Pirate King. It was shown to elicit a similar, if not a stronger, reaction from most people. So, it is reasonable to believe such a conclusion, especially with the given arguments I have just provided.
After that, Sabo & Ace begin finding the intention of becoming ‘captain’ a problem to them since all three of them share it. This means that what had been just shared by Luffy was the intention of becoming ‘captain’. Given both this and the past repeated declarations of Luffy to become ‘pirate’, it logically follows that what Luffy desired to become was a ‘pirate captain’. That’s partially what was said there.
At the coliseum of Dressrosa, Barto told Sabo that Luffy is the future ‘Pirate King’, and Sabo said that he has known that since a long time ago. In other words, Luffy told Sabo his dream of becoming the Pirate King sometime in the past, and the time you linked us to provides the perfect setting for such a declaration to be revealed, given both the fact that Luffy hasn’t seen Sabo ever since that time he was supposedly killed & the strong suggestions that what he shouted in that place was his ambition to become a ‘pirate captain’.
There is nothing odd in the author’s omission of Luffy’s words regarding his declaration of his ambition. That is a literary device called ‘ellipsis’. You omit a certain event(s), and let the reader fill out the gap as he or she reads the story. It could be used either to save time or for stylistic reasons. I believe it was the latter.
If he did indeed say something else, it should then be an addition to that, not a substitution for it.
Concerning the argument that what Luffy said here meant becoming Pirate King, that’s not true.
What luffy said there is that he’d be ‘a great pirate’, and he preceded it with ‘I’m gonna be stronger than anybody’.
If being stronger than anybody meant being the Pirate King, then, by this logic, Whitebeard should have ruled the Grand Line as the next Pirate King after Roger, given that he was considered by everyone (including & especially by the author himself) as the ‘Strongest Man in the World’, but he wasn’t a Pirate King, despite reigning supreme in terms of strength.
When Doflamingo was being taken into Impel Down by a naval ship, he said that Whitebeard never took Roger’s throne, but instead ruled in front of it.
This proves that standing supreme in terms of strength does NOT make a person Pirate King. Being the Pirate King means far more than just being stronger than anyone.