Fortnite started development back in about 2011 with Fortnite: Save the World. After it was released in July 2017, they shortly after released a battle royale edition of the game that took off in popularity. So, Fortnite Battle Royale was just a gamemode with the cartoony look and building mechanic of Save The World and put it in a battle royale. So, no, Fortnite is not a copy of PUBG.