Kuzan, better known by his former epithet Aokiji, is a former Marine Admiral. He was nominated by Sengoku for the position of Fleet Admiral, however, 2 years ago, after losing the position to Akainu, he resigned from the Marines.
He has currently affiliated himself with the Blackbeard Pirates for unknown reasons.
So how did Kuzan get a copy of the Final Road Poneglyph??
It comes down to its location which is Mary Geoise.
First let me explain how it ended up in Mary Geoise.
Gol D. Roger deeply cared for his crew and he knew the consequences of discovering the Truth about the World Government and the Void History. In reality the Roger Pirates knew more about the Truth than the people of Ohara.
I can tell you everything about the world.
Roger’s right hand man “Dark King” Silvers Rayleigh openly stated that he knows everything and was willing to explain it all.
So why were the people of Ohara killed but the Roger Pirates alive?
Rayleigh is a Legend but the rest are not as powerful as him (Shanks and Scopper Gaban being exceptions) and wouldn’t be able to hold off an Admiral.
Why didn’t they do the same to the Roger Pirates?
Roger traded the Final Road Poneglyph to the World Government and in return his crew gets to live out the rest of their lives without being hunted down.
They are famous pirates so they still have active bounties but there are instances like
This isn’t the first time I’ve heard about sightings of him
Marines keep tabs on Rayleigh and possibly the other Roger Pirates but they never act on it unless they interfere. Kizaru also just left after Kuma dealt with the Straw Hats because higher ups like him know about this deal and Garp made the lower ranking Marines avoid conflict beacuse of the war with Whitebeard.