From wowhead
The tweet clarifies that, whenever you unlock World Quests in Kul Tiras e Zandalar, your whistle will be upgraded to work in the new continents. Players that don't have the whistle, because either deleted it or didn't play in Legion will receive a brand new Whistle that works both for BFA and Legion zones.

In order to unlock World Quests in Battle for Azeroth you must reach friendly with all faction-specific reputations, Proudmoore Admiralty, Storm's Wake, and Order of Embers for Alliance and Zandalari Empire, Talanji's Expedition, and Voldunai for Horde. You are also required to unlock all basic outposts in each of the enemy zones.

This unlock is account-wide, meaning all your alts will automatically have access to World Quests at 120 after you unlock them once.