Custom license, not free for commercial use. Assets are still proprietary. https://github.com/flibitijibibo/RogueLegacy1/blob/main/LICENSE.md
I guess the main benefactor will be the modding scene. Indie devs can make deals with Cellar Door Studios to get a commercial licence with additional conditions. It might be close to open source in practice if the commercial licences are handed out generously.
Makes sense to protect the studio’s reputation against potential droves of crappy, barely altered clones with scam ads.