I can only find the scans in Kissmanga. You can check the official by your own means. Or just recheck the anime scene for those, I did check the anime, and they did the same with giving them the title of "Great Pirate". Anyway, those are the panels:
If I'm not mistaken even kaido was called a great pirate as well. I'm just too lazy to check where it was.
The point is, Great Pirate is given to the person who reached the world summit. It's the point where you are one of the big sharks that are directly big candidates for ruling the sea. In that sense, the 4 yonko are the best representative for that title. Naturally, in the past, Roger was a great pirate before being PK. And Whitebeard despite being at the pinnacle of his strength, it was said that the members of Rox 3 years after their captain disappeared, that they started to make a bigger name for themselves. So, in that sense, WB started to make bigger name for himself as a CAPTAIN just recently. Which is why he is YET to be considered a great pirate in the introduction box in that flashback.
I like the details in that because Oda even didn't neglect the detail of Roger being called "Gold Roger" and NOT "Gol D. Roger". He made sure of it, because Roger at that point was still clueless about the void century and D. stuff