Spoiler One Piece Chapter 1167 Spoilers Discussion

So Shanks reached a bounty of 1 billion in less than 2 years and became a Yonko 8 years later
But Oda made a big mistake here. Oda had previously shown Shanks at 20 years old traveling to yassup's island
Am I wrong or did Oda make such a big mistake?
The implication is that he abandoned piracy at that point, not that he couldnt have been a pirate beforehand. I am convinced the only retcon he was given the 1 billion at 14 years ago (thats when he is no longer a Blade of God it seems), because then 8 years later he becomes a Yonko. Any other information isnt directly from the manga.
 
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