Maybe not compared to a free game like Fortnite, but 50 million copies is huge when it comes to video game sales. The 50 million number is from Xbox and PC only.
GTA V is the top grossing game of all time and sold 90 million copies on 3 platforms. Minecraft has sold 150 across way more platforms and has been around way longer.
They're different pricing models. Fortnite is free to play and PUBG has an up-front cost. Copies sold/downloaded or peak active player-base is probably a more relevant comparison.