User blog comment:Gametroller Prime/Four Great Powers?/@comment-4401815-20160218030825/@comment-25501246-20160218225114

Yeah, he became a favorite because he stole Whitebeard's DF.

He only became an Emperor after he started reconquering Whitebeard's territories, as stated by Jinbe. He definitely wasn't an Emperor when they were running away from Sakazuki at the New World.