Yeah, I know that. Back when fans were speculating whether Sabo was still alive, they were wondering if he met Robin. Ever since chapter 731 revealed the truth about Sabo being alive, it was confirmed that Robin, at least, knew Koala... Until now.
She didn't knew about Sabo story because he didn't told her. Or he told her but she'd promised him not to tell Luffy until Sabo come to reveal himself to Luffy. ^^ So complicated, but Oda will show us the true story.