Sengoku said that Xebec had touched upon knowledge of too many taboo subjects. That's the kind of thing Gorosei want the World to least know about .
True but it look like roger and garp along with god knights take care of it for them and i doubt rocks had anything set up that he could tell world about that info .
To me it was always interesting how lucky they were that roger or the rest of his crew did not tell the world any secrets . ( i sure morgans would love print a interview from the PK or his crew with that info lol )
I always wonder if because they knew they were to early and that giving out info would made things harder for next person when the time came so they keep quite .