A) Gearbox bought the Risk of Rain franchise a year ago
B) Gearbox published Risk of Rain 2 and co-developed the first DLC, so this isn't their first rodeo in the franchise
C) Gearbox announced a DLC for Risk of Rain 2 the same time they announced the mobile game, and they claimed they will support the game for many years to come. Their first thing wasn't "immediately making a mobile game".