It was already mentioned in the manga, just about 4 years ago
He was a rookie at the time, hence he didn't have the tattoo considering he didn't even meet Jinbei / Whitebeard at that point
For his knowledge, he was simply thinking of Kid, Law and Bege in their respective seas. They didn't travel through grandline yet, except Cavendish who had a year start and he clearly stated Luffy had yet to start his journey but that from expectations, he already knew he'd cause a storm