This is absolutely not true. The unified UI - what literally brings the whole package together was developed completely in-house. Under the hood this includes major systems such as campaign playlists, save file management, campaign management, user content management, and matchmaking systems, including both dedicated server and P2P code. And of course the UI for all these systems. They also completely manage multiplayer playlists and their settings in-house as well.
That said, the team the developed the universal UI and put the games disparate pieces together is a relatively small side-team that largely did not overlap with the Halo 5 Guardians team (who is working off of completely different multiplayer code on all levels, so the matchmaking issues shouldn't be present). Even beyond that its issues are irrelevant to H5G, as they largely stem from an apparently all to brief dev. cycle, no less than four developers working on the project at once, and the sheer complexity of the project mixing together to form a major clusterfuck.
