User blog comment:Kaido King of the Beasts/Chapter 908: Imminent, Imbalanced, Improbable Whims/@comment-3185063-20180615081043/@comment-6050542-20180615160815

Well Lafitte was easily able to infiltrate Mary Geoise. Maybe Lafitte has infiltrated the WG before and has access to questionable info. So Blackbeard might have found out about Im through him, assuming of course he actually knows about Im.