who knows maybe we'll find out if he has any connection to WB in the flashback
Doesn't make sense though. Whitebeard never left the shore of Wano, and never visited after (Marco explains this to Ace, they didn't to avoid War with Kaido).
Also, wasn't the Lurking Legend supposed to be the "greatest" enemy Luffy will ever face? Ushimaru is a villain?
Imo, it's just the return of Rocks. Either this was metaphorical (i.e. Just Kaido and BM together), or Rocks is legitiately returning in the story somehow. While Blackbeard might be his living will (its a theory), he's already been introduced. Im can't be him because he's seemingly not related to Whitebeard, but who knows.