Yes it is. Owning the IP doesn't translate to first-party when discussing studios and their design ethos; both State of Decay and Quantum Break translate to those developers being "2nd party partners." Halo Wars 2 that OP mentions would also be in contention. But that one is more of an amalgamation since 343 (first party) and Creative Assembly (third party) share developer credits there.
The game is first party if Microsoft owns the assets, doesn't matter who works on it. There is really no such thing as second party. You have first party games and first party studios, then you have third party games and third party studios. A third party studio can work on a first party game.
“Today, State of Decay was given a Refused Classification (RC) rating by the Australian Classification Board," said Microsoft, in a statement sent to Kotaku, "meaning that the game cannot be made available to Australian customers at this time. Microsoft is currently evaluating the options with regards to the title’s classification.
"Microsoft operates within the legal requirements of the Australian Classification Board when it comes to the rating of all its first party gaming titles and agrees that not all content is suitable for all audiences. The Australian Classification system plays an important role in ensuring that Australians can only access age appropriate games and content." http://www.kotaku.com.au/2013/06/microsoft-confirms-that-state-of-decay-has-been-refused-classification/
Shannon Loftis: I’m the head of Microsoft Studios First Party Publishing. That means that: number one, I have the best job in the world; and number two, I manage a team of about 90 game coders and designers who partner with independent game developers all over the world to help them bring their games to our platforms.
Undead Labs are industry veterans. Jeff Strain worked on World of Warcraft and then at ArenaNet – he’s not a slouch in game development. When he told us he was starting a studio and they had the pitch for State of Decay, he said they had a very specific way that they wanted to approach the development. So we started out with what was essentially an XBLA level-sized game, a smaller game, and then we did some very deliberately thought out DLC packages to sort of test consumer response, rather than building the entire thing. State of Decay plus all the Year-One Survival Edition DLCs in one release would have been a big risk to take, so we did it as a modular release. And now with State of Decay 2 we’re taking much of what we learned from the first game and we’re making it a triple-A game. http://www.gamesradar.com/microsoft-studios-general-manager-shannon-loftis-interview/
Xbox Head of First Party Publishing, Shannon Loftis, spoke with Geoff Keighley recently on the E3 2016 YouTube stream. She told Keighley that there are quite a few exclusive games that have yet to be announced as more are in development.
She said: “I head first party game publishing for Xbox One or Windows 10 or any of our platforms. ‘We right now have, i think, 14 games in development. We talked about four of those this morning – Killer Instinct, Scalebound, State of Decay 2, and ReCore. We also have Crackdown 3, which we announced has now been moved into 2017, and there are a few other ones that aren’t quite ready to be talked about yet.” http://attackofthefanboy.com/news/microsoft-unannounced-xbox-onewindows-10-exclusive-games-way/