Have you even read the flashback
Both the name One Piece and the title King of Pirates is something the world started to call him.
Even if we assume the WG called the treasure that from the start, which we have no indication of, Roger did not know about it.
Entire flashback they called it "the treasure".
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See, let me tell you something.
It's sound silly but impactful.
It's a dream
And, it's very important from story point of view because oda omitted that out
There are only three notions which actually fits the criteria.
1)Luffy unknowingly said the full form of "D"
But it's far fetched and leaves lot of room for interpretation as of now.
2)Luffy says something's on the lines of "I want to be free man" - silly for a child to say and impactful.
But no reason for oda to omit this out.
3)this leaves us with one piece. Now, we can safely assume one piece is not some treasure world government is trying to hide.
One piece could also have a different name all together - a more familiar term
Now, yes World called it one piece and I missed this part but world government still knew about one piece or whatever this is even before roger became PK.
And, why exactly the world named it "one piece"? Who came up with this idea?
Oda can literally twist the narrative to his liking
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It will be king of the world.
There is no reason for oda to omit this one out like he did