They probably have a license from Hasbro for certain content - likely just the Player's Handbook - which would limit e.g. which playable races, subclasses, and spells they can use.
Gith are from Tome of Foes. I'd be surprised to not see additional FR based content in the game.
Fair point, I forgot about Gith. Regardless of whether the license is exactly for the Player's Handbook or not, they probably have fixed parameters for what they are allowed to include.
