These software managers don't care if the games installed are legit or pirated, because they would be all externally installed games. They only care about games installed through them, e.g. games downloaded through Steam into a Steam library (folder). You can use Steam to link to any external games, and all your friends would see is "xxx is playing third party game: Skyrim" or whatever. They wouldn't have any Steam-bound features like connecting through Steam friends lists or connecting to Steam multiplayer servers because they are running outside of the Steam DRM environment, but if the game connects using third party servers (like I assume Garden Warfare does since it's cross-platform) then I don't think there would be any difference.