Still, this is just a single mention of tvOS in Fortnite's code and shouldn't be taken as a guarantee that the game will be available to play on Apple TV soon. If it does, the game would likely require a compatible wireless gaming controller, two of which Apple sells for $49.95 on its website.
Fortnite Battle Royale on iOS is free-to-play (like it is on all platforms), but due to in-game content purchases the app in June reached $100 million in revenue 90 days after launch. Fortnite initially launched wide in summer 2017 on consoles, macOS, and PC, focusing only on the Save the World mode, with Battle Royale releasing a few months later in September 2017.
Update: According to Epic Games, there are no plans to bring Fortnite to the Apple TV. From Epic's Senior PR Manager: "Epic isn’t planning to bring Fortnite to Apple TV. References to tvOS in the Fortnite files are the result of general Unreal Engine support for the Apple TV platform."