@Audity:
well, Shanks isn't exactly with Sabo/Ace/Luffy in the scene that mimics this chapter's scene, so it's obvious that Shanks is referring to just the usual Pirate King phrase.
As if we saw every piece of interaction between Shanks and Luffy in the times they were together. There was loads of space for Luffy to have said things to him we haven't heard yet.
And if the concept of us not having heard everything yet sounds ridiculous, consider this moment:
I sure don't remember Luffy mentioning Sabo way back in chapter 1, in this very same scene where he's just setting sail. And that's because Oda carefully hides information from us until it's become relevant.
If the only thing Oda wanted us to notice was the drawing styles, there was still no need to censor what they said.
Luffy still has mysteries going for him. All we knew up until chapter 506 (FIVE HUNDRED AND SIX) was that he wanted to be Pirate King. And then boom, more than halfway into the total of chapters we have now, we found out that he wants to be the freest man on the sea, and that is (part of) what being PK means to him. There is no reason to assume that we already know everything at each point in time, and the way Oda handled these scenes are too suspicious to not lead into anything meaningful down the line. He doesn't need to have said anything ground-breaking that would completely give us a 180 on his personality so far. Just something that Oda doesn't quite want us to see yet that will enable Luffy to, as WB said, "carry centuries of history upon his shoulders" and make use of the info on the Poneglyphs in the present somehow.